[donut]

DD, comfortable, and gas mileage. No "wheeling"

My .02, put the money toward a good tune and possible dizzy re-curve. Possibly a "rv" cam, for a little more efficiency. I personally don't remember if an '85 would have a rudimentary computer system or not.??

Comfortable, that's up to you. One persons comfort is another mans life sentence.
Mileage, don't expect much. I really would expect 12-16. Higher great. lower, find out why.
DD and mild off road. I think GM didn't do too bad...

Keep in mind, you are dealing with some parts that are 25+ years old.

Gears, until you know what you have and play with tire size and a gear ratio calculator, change what's broke... Used trans is like Russian roulette. Used may work great, or need rebuilding asap.

This post is IMO, your goals don't seem unrealistic for what you already have. I suggested different truck if your goals were more into the heavy wheeling. I'm also not into spending your money for you.
Rebuilding brakes, driveshafts, changing seals etc will eat up a budget quicker than you think.