Are you planning on doing suspension mods and building a trail rig? Or plan on keeping it stock?
If you are going to mod it, I would recommend going for an 88-89 model, instead of the 84-85. There are several advantages to the newer ones that the older ones will need changed.
A newer one will need an SAS performed, but you will already have fuel injection and the desired steering box in place. It will be a 4-5 year newer vehicle. The rear axle is wider- most people with the earlier models look for an IFS rear end to swap in. I wouldn't bother with a V6, as the potential of the 22RE is far greater (and cheaper) to build. The newer models are also easier to find, so you can really shop for the right one.
With an older model, honestly, the only advantage is that the solid axle is already in place. If you are doing a lift, you will still have to tackle all the steering problems, including finding a box to swap from a newer runner. You will eventually want to swap a newer rear end on. Fuel injected ones (only available on SOME 85s) are getting pretty hard to find.
As far as rust goes, make sure to check the body mounts from the inside and underneath. I've seen a few that rusted on the passenger A-pillar- open the pass side door, and inspect the pillar about halfway up the windsheild (where the door seal sits). Everything else is pretty much easy to see walking around the vehicle. Tailgates can be bad.
Make sure everything works, including the rear gate window. Look for an SR5 model, which will have nicer interior and a tach. I've heard that Toyota automatics are some of the best, but I've only owned manuals. My '01 Tacoma has an auto, which works very well, but I don't know about wheeling with one.
Don't plan on getting too great of MPG, especially with larger tires, etc. Most people I know who've actually checked get between 12-15.
