we were considering building one a few years ago, before I rebuilt the loader on the tractor and put forks on it, and I was able to find a used cross country fork lift,
but this much I can tell you, as we had to completely rebuild the mast of the fork lift we did buy,
the rollers on our were wore out and one was missing when we got it,
at first it looked like it was just a large bearing on a stub shaft, and in a sense it is, but the bearing has a sleeve over it, (now ours is a big lift), and the wear and abuse of my son using it to load scrap iron as a living for three years was hard on things, but the roller has a bearing in the center and then a mild or slightly harden roller over the bearing, we turned these out on the lathe, I think if one just used a bearing with out the wear sleeve one would crack the outer bearing shell,
when I was considering making ours my thoughts were to use channel iron and then use a tapered bearing sleeve, to fit the inside of the angle, but I think after having a "real" fork lift I would put in wear strip and make the surface a 90 degree and not the light taper on the channel iron, or use some thing like truck channel that is square,
If you make the riding inside track and the lift chains, use heavy chains, and the proper type, (it is no fun to have a few thousands pound in the air and have a chain break), and it scares the stuffing out of one, (fortunately no one was hurt), the chain on our fork lift had been abused and improperly mend before we bought it, we ended up replacing the main leaf lift chain (bought it from Baum iron company),
on a three point when we were considering it the thought had crossed our minds to put some out riggers on it and then some trailering wheels that would carry the load instead of it all on the three point,
(the loads we were dealing with heavy and old fork lift, lift capacity were up to 8000 pounds, and the lift we have lifts up to 30 feet, and is 4 wheel drive), (there were a few times we had loads it would not lift), and do not lift the load any higher than nessary,
the ones in the pictures below are about 1500 pound lifts,
two net pictures of some three point lifts,
3-point forklift-Selling Leads-Townsunny Implements Sales Dept.
http://www.best-tractor.com/img/3-point-forklift.jpg