These trucks 40/60/62/80 get about 12-14mpg on average. Don't expect more. I have owned my FJ60 for 3 years and it now has 130K on it. Got it with around 100K for $3200 in very very good condition 100% stock. This is considered very low mileage as there are many many on here with 250K on the original engine!
I have replaced the battery, master brake cylinder (easy to do), changed fluids, and had a shop replace my throwout bearing as it was making a terrible sound ($700).... but the clutch was fine?!?!?
The 2F engine is basically a tractor motor. Low revving, with ok torque, and reliable as all Toyota motors. I would get an FJ60 simply because they are a manual trans and have the classic round headlights. If you can afford to put an H55F (5speed) behind it or find one that has it already they really could use an overdrive. If not put a lift and 33's with the stock gears and it has overdrive now

I run the stock 4speed non OD trans. (they have 28.7" tall stock tires). Just a little doggy in 1st gear with the 33's. 31's fit with stock suspension BTW.
I have driven my truck at 7,000ft and it performs fine. Certainly no barnstormer... but these are trucks... not race cars. If you want more power buy one with a V8 conversion. It will get about the same gas mileage... but one with Overdrive would be preferred.
A twin blow up mattress in my FJ60 fits perfectly and it can carry a ton of stuff. If it has over 100K on the truck and is stock suspension... plan up putting on new springs/shocks or it will be a bouncy/soggy ride. I have Old Man Emu suspension (OME) and love it.
I have a performance car and then my adventure vehicle (FJ60) like you. Funny that my 500hp car gets 24mpg+ on the freeway and my 125hp FJ60 gets 13mpg.
How about one like the one I attached!
Good luck,
Mark