OKAY!!! So the lecture begins!! You like old vehicles you say?
CJ's (Mostly unmodified) ride like it has no suspension, so if you want a stock, take that into consideration. Also, seeing as most of the CJ's you are going to find are about 30 years old, you typically find crappy wiring, old plumbing, sometimes drum brakes on all four corners, and rust. Lots and Lots and Lots of rust. Check EVERYTHING for rust. Floor, under seats, front clip, frame, shock mounts, engine mounts, EVERYTHING. Oh, if it has carpet, its 100% certain that there's rust. I can go into the basic things to check on all cars if you want, but im thinking you already know that.
If your wanting to offroad, then CJ-7's are better in most cases because of the longer wheel-base, The CJ-5 has a shorter wheel base and are more prone to turnovers.
For some years of CJ's AMC made a V-8 engine for the Jeeps, a 304cu, or 5.0L. Also there was a straight 6 258cu, and a 4 banger, but i dunno what size it is. they're all good engines, but obviously you want the 6 or 8.
Some good signs are new brake lines, fuel lines, front disk brakes, newer carb, the 2 bbl or even 1bbl obviously are not very good.