Welcome to the Isuzu side Tres. There's a lot of Trooper folks running around who can probably give you more complete feedback on their experiences, but I can offer some generalities.
For the money you're talking, you'll probably be looking at either a later first generation Trooper (89-91 or so) or an early second generation Troop (92-96ish) depending on location, mileage, and other good stuff like that.
With the first gens, you have either a 2.6L Isuzu-built I4, or a 2.8L GM V6. Generally, the two are pretty comparable in performance overall. The 2.6 has some power options, while the 2.8 is a favorite for a 3.4L transplant. With this, you either get a GM sourced autobox, or an Aisin manual. It's all in what flavor you like. Of course, you also get a brick on wheels, so the usual aerodynamic penalties of such a thing come into play (mpg, wind, etc.).
With the second gens (my personal suggestion), you're looking at basically the same transmission options, but the engine switches to an Isuzu 3.2L V6 in either SOHC or DOHC flavors. It's a decent powerplant, I have one in my Rodeo, but again you're dealing with the aerodynamics of a rounded brick, so mpg is still going to suffer. The second gens have a lot more going for them in terms of creature comforts and refinement, but they're still built really solidly. Of course, that comes with a weight penalty, so see the mpg commentary above.
Things to watch for:
-2.6L 4s are susceptible to blown head gaskets/warped heads if overheated. The bottom end is very, very tough, but it's something to keep in mind if you go for the older trucks.
-3.2L 6s can have some issues with small oil leaks in some years and lifter tap. Each are readily dealt with, but again worth keeping in mind. These engines are also known for being misdiagnosed with rod knock when it's really just the timing belt tensioner, which is a whole lot less severe an issue.
That's just a glancing shot, but hopefully it'll give you something to start with, and others will chime in soon, I'm sure.
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