The manual transmission in the Jeeps from '87-89 is the Puegot BA-10. Its about worthless. If you plan on 'wheeling it, plan on replacing that transmsission in about a year.... "Been there, done that" The automatic in Jeeps has always been a tough unit. Also, the axles aren't very strong either. Strong enough for a stocker though, if you dont lock them (althoug some will argue against that, too). The engine is the 258, all choked up on emmisions crap and running a horrible carberator. Take it off-road and will cease to idle, garaunteed, until you figure out where to spray the carb cleaner. If your going to buy a YJ (87-95) Wrangler, look for the '91 and newer models. They've got a great engine and a stout drivetrain, even if the axles are still weak. In '93 the rollbar was improved, but thats nothing to rule out an older model on. In '95 the front axle got bigger, stronger U-joints, but those are probably out of your price range.
So...'91 and up, or '87-90 with automatic if you dont mind doing some work to the engine to make it a nice runner. I've got an '89. I blew out the BA-10 and replaced it with an automatic thats bulletproof. I've ripped all
the vacuum lines and wiring off the engine, tossed the computer, replaced the carb and ignition system, and am currently re-wiring the engine bay....
Make it up as you go.