unless you're looking for an huge engine/harness swap, get a 4.0L from the start (that will eliminate the '86 MJs). rust free is the most important thing to me. floors are always suspect no matter how nice the outside is (they are prone to leaks getting under the carpet). obviously the condition of the bed matters little to you. they are trucks, so the bed can get a bit beat up over the years. 87-89.5 came with a weak stickshift, but it's easily swapped out for the later ones. essentially they got a little better each year, so all things equal I'd probably take a 91 over an 87, but I'd never turn down a good deal on an 87. These trucks are like Legos. you can build whatever you want. even 4wd bolts right in to a 2wd without modifying the body.
there's a thread at the top of this forum about the "most important tips" that will have lots of good, um, tips.