I took as my model the dimensions of the power racks I saw for sale in the Power Systems catalog.  Dunno what is on their site.  I just took the URL off the catalogue.  I also looked at those available at Body-Solid.  Don't have their URL handy.  My aim for dimensions was 92" high, 44" inches between posts, left to right, and 32" front to back.  Those seemed to combine the biggest of what I saw.  I cut scaffolding pipes as follows:

4 posts, 92" each (not really--see below) 
3 side-to-side braces, 48" each
6 front-to-back braces, 36" each

22 fittings for joining parts together and for racking the barbell.  Most of the fittings I used were 'doubles', which clamp two pipes together with a hinged clamping bit that is tightened by a nut and bolt on the opposite side of the pipe from the hinge.  One clamp for each pipe integrated into a fitting. There are also 'single' or 'lap' clamps which I've used in a couple places where the pipes stay stationary.  These grip one pipe with a curved metal bit that resembles your hand when you use a hook, thumbless grip.  Tightening the nut on the bolt on the other pipe
pulls the two pipes together.  At a minimum I would want 8 doubles.  4 for the racking pins and 4 for the safety catch bars. The others aren't so important.

Think of the rack as 4 rectangles: left side, back, right side, and top.  Help assembling the rack probably isn't necessary, but the help of my 12 yr.-old daughter was appreciated.

Begin with the left side (for ease of describing the process). Take one of the 92" posts and at the bottom attach a 32" pipe on what will be the outside of the rack (to give you maximum space inside the rack).  Approximately 12" up from the bottom of the post, attach one of the 48" pieces. This must be 90 deg to the 32" piece, toward the right side.  The 48" piece will run across the back side of the rack.  Now take another 92" post and attach it to the end of the 48" piece at the same height it attached to the first post.  At 90 deg. and running toward the front of the
rack, attach another 32" piece at the bottom.  Attach one 92" post to each of the free ends of the 32" pieces.

You should now have 3 upright 'U's forming a 'U' on the floor. Complete the 4 rectangles. I put a 48" pipe across the back right at the top of the back posts.  Then one 32" pipe on each of the left and right sides just below the back piece.  Then just below those was the level I used for the front 48" piece.  This was just the right height for me to use it as a chinning bar. I'm 5' 7".  Those shorter can obviously place it lower, and those taller might well want to place it at the same level as the back pipe, at the very top of the uprights.  Alternatively, put the 32" pieces at the top and the 48" pieces just below those, giving the in-between height for the chinning bar.  All of these attachments
can be permanent.

At this point you have everything together except the bits the entire rack is to support: the adjustable safety catch bars. Attach a fitting at each end of each of the remaining 32" pieces. If you are using double fittings, these attachments will not needto be moved. Now you can attach these bars at any height you like up or down the sides.

There are 4 remaining fittings.  Remove one of the nut/bolt from each.  These can function as the racking pins where you will rack the bar for squats and bench.  Just work out where is best for your height and place them there.  I've used 4: 2 for bench and 2 for squats.  You could do without these, if you are willing to begin all your benching from the safety bars as well as all your squatting.  If forced to choose, I'd do that for benching, but not squats.  Or you could use just one set, mark the proper heights for racking squats and benches and move the fittings up or down as needed.  Prior to assembling the whole thing, I slid 2 large rubber bands over each of the rear uprights so that when
one set is not in use, I can use the rubber bands to keep the clamp out of the way.  One on each side would have sufficed; just move it to the one you don't want to use, freeing up the one you do want to use.  Probably not much of an issue any way.  Just check and make sure that the double fittings you use for this purpose don't open more than 90 deg. The curve in the clamp works nicely for holding a bar.  I can't recall how strong the fittings are in this use, but I seem to remember that I had nothing to worry about until I was out-benching Anthony Clark, which isn't
any time soon.  It is worth asking your scaffolder details, though, since things may be different where you are.  There may be scaffolding fittings that are made to function as hooks, even at angles like regular bench or squat racking pins.  See what's available to you.
 
 

Finer Points:

When assembling, make the fittings snug, but not very tight.  I put mine together without a square, so I needed to fiddle things to get the angles right.  The fittings take care of most of this, so you can't get too far out of alignment.  The fittings tend to pull things to right angles.  Those with proper square angles to use as a guide, shouldn't need to do much adjustment.  Just be sure to tighten down all fittings when you have everything in proper alignment.

An aesthetic point.  When I was getting advice, I was told that you will lose 2" on each side of the cross pipes, due to the overlap of the 2" pipes. True.  My fittings have another inch past the hinged clamp affair.  If you want to run the pipes so that they are flush with the end of the fitting, then make the cross pieces 50" and 38". Or, just sacrifice two inches from the inside dimensions of the rack.  That still gives 30" between the front and back posts. Plenty, in my experience, though I've not tried lunges in the rack.  As it is, I opted to give myself the extra space inside the rack and didn't run the pipe to the end of the fitting.  This really is an aesthetic point and not a structural issue.  Of all the fittings I had only 1 actually provided extra contact between the fitting and the pipe in this extra inch of fitting. For all the rest the only contact between the fitting and the pipe was the area covered by the clamp.  The rest of the fitting served simply to secure the bolts.

At first I tried marking the places for the catch bars with pencil.  I found it a bit fiddly to get the pipe right where I
wanted.  At the moment I'm using a length of wood cut to length so that now I can just rest the pipe on the stick and tighten the fitting on one side and then do the other.  It's still a bit fiddly.  I think I will just make a 'U' out of scrap wood which will allow me to set the 'jig' down without holding a stick in my hand.  Then I can rest the pipe on that and tighten the clamps. One jig for bench, one for squats and one for seated shoulder press.  I don't have rubber mats, so I also have one for deadlifts.  I figure that the height I need to just keep the plates from hitting the floor compensates for the fact that I deadlift in running shoes rather than slippers.
 

Ease of use is not quite the same as commercial racks which permit adjustment of the catch bars simply by pulling bars out of holes and sliding them in different holes.  Mine requires a wrench (spanner) and undoing and redoing four nuts.  No big deal. I consider it a minor trade-off considering how little this setup costs, but you might put a very high premium on changing bar placement in 30 sec. rather than 2 min. while you catch your breath between exercises.

Safety should be your main concern with a rack.  I cannot quantify any comparisons.  I know that I'm using the same materials as commercial scaffolders who erect scaffolds several floors high.  The fittings are rated for 2000 or so pounds and the pipe for more than that, I believe.  It is surprising to me how easily the fittings bite onto the pipes and if one side is properly tight and the other isn't, the snug fitting prevents the other from sliding any significant amount.  If it does slide a bit, then at some point it starts to re-grip the post.  It's almost impossible to have both sides loose and then have the cross piece slide down very far without catching.  As long as you haven't left several fittings very loose, there should be no problem with the entire structure crashing down over your head. Obviously, the best idea is to simply tighten the fittings properly, which requires no special amount of strength. If you want more specifics, ask your local scaffolder.  My weights are pretty puny compared with many on this list, but I'm convincedthat the cross bars and the rest of the structure are good for much more than any of us likely to squat or bench.

If you really wanted the slide-in/slide-out convenience of commercial, perhaps a proper drill press could be used to put in holes.  How that would compromise the strength of the uprights, I can't say.  I don't have access to such a press, so it is not an issue for me.

If you are very tall and need more than the 92" for your chinning bar, just make at least one pair of posts taller.   FWIW the posts don't need to be the same height.  My rear posts are 92" but the front are more like 100", since I didn't feel like cutting off the remaining eight inches.  So, it would be easy to make the front taller than the rear for a higher chinning bar.

If you are in a space that doesn't allow 92", make it shorter. The Body-Solid racks are only 84" tall, I think.  Chins with bent legs are still do-able at that height.  If you don't care about chins you could easily get by with something only 5'-6' tall, since the real work you want done by the rack is done by the catch bars.  For squats, the height for those need to be only 30" or so, depending on your height.  5'-6' at least on the rear posts gives you enough height to rack the bar for squats.  The front posts could be shorter, with one significant note.  Going for such a minimal rack would mean that the safety bars are actually helping to keep the structure together, unless you put the 'top' cross pieces below the catch bars.  The way I've copied commercial structures, the entire structure stands independently of the safety bars.  Better, I think, but if you are short of piping, the minimal structure should still be all right so long as fittings are tightened properly, which is trivial.

The 48" pipe that forms the bottom of the rear rectangle is about 12" off the ground.  Move this up or down as you need so that you don't hit it with your leg when you (un)rack for squats.  12" works for me, so I've not moved it.  Can't imagine that you can't find a height that would work, but in that unlikely case, make the bottom front-to-back bars longer than 36", then connect the 48" to the ends of the bottom pieces that protrude out the back of the rack instead of attaching it to the posts.  Give yourself enough space to accommodate your feet as your step forward to
(un)rack for squats.  My set up saves space, since you must bend at the ankle and knee anyway; just set the rack a couple inches out from the wall.

I think this covers it.  Don't let the expense of a commercial power rack stand in your way of working out at home, if that is what you'd rather do.  I hope that this description is clear. I'm happy to answer what further questions I can.  Just remember that I am _not_ a scaffolder.  If you need technical answers, you'll need to go elsewhere.  I'm just someone who was too cheap to buy commercial and found what I think is a very good alternative.  I enjoyed doing the work myself and I'm not particularly capable when it comes to construction and fabrication.  I've no special tools and no special mechanical or industrial skills, so unless you have trouble boiling water this project should be within your capabilities, esp. if you have the pipes cut to length for you.