Matt, I'm a little confused here. You start out talking about building just a drivetrain and waiting for a car to come along to put it in as funds become available. Now this guy your brother is talking to with "a few of them", are they drivetrains or complete cars. I would say $5K for a complete running car with 130K miles could be a pretty good deal, provided it's not trashed. But $5K for just the drivetrain with that many miles seems a bit spendy.
If we're talking a complete car the first thing to look for is rust. Unless it's been well taken care of, a 25 year old car is likely to have some. Look for upgrades, i.e. turbo, chip, exhaust, intercooler, injectors, fuel pump.....the list goes on. Has it been raced? T-tops are usually less desirable than hardtops. If the car is a T-type or Turbo-T it may not have a posi rearend. Look for a G80 on the options code sticker under the trunk lid, that's a posi. A GN will have a posi. What other options does the car have. Then like any used car look for the usual stuff, does it smoke or miss, how straight is the body, condition of the paint (repainting a black car can be expensive), how's the interior, is the headliner falling down (this is a problem with many TR's)
Another guy who might help is Mike in Spokane. He is m233roller. Do your homework and read all you can on this form and others. Good luck!