I think you'll be lucky to get 15. I get around 15 in my 95. I used to have an 89 FJ62 (same engine and tranny as the 92) and I was lucky to get 13. I usually got 11 or so. That was an arguably lighter vehicle with part time 4WD.
You're right about the price though. From what I've seen on 91-92's is that a super nice one shouldn't be more than $7-8K with average condition ones bringing $3500-5000.
I doubt you'll have any reliability issues with it, especially since it sounds well cared for, but mileage will suck.
Oh yeah, it'll be slow as hell too. <img src="/forums/images/graemlins/smile.gif" alt="" /> But I think the 91-92's are perfect candidates for a 350 swap.