You didn't mention whether diesel was a must have, but there are bound to be a few International shorties out there with 6.9s, 7.3s, DT360s, DT466s, T444(E)'s. Some have the 9.0, I've heard some people bash these, but not really sure why. Obviously the AT545 is to be avoided for highway or mountain use.
Most of the older schoolies I've seen were gas, albeit my school days spread across the 80s into the mid-90s. Most of the buses in my area were the old step-van-grilled bowties and GMCs with alligator hoods. A few Fords of the same vintage, even a Dodge or two. All were good, we rarely used spare buses or broke down. We were just starting to get diesels in the early 90s, most of which were Fords.
Fords aren't bad if you can get one with air brakes. Avoid their hydraulic brake systems at all costs. Hard to find parts, problematic at best, and you'll be hard pressed to find a shop that will touch them. I know where there's a 1982 Blue Bird on a Ford chassis with an 8.2 Detroit and automatic trans for $1200 or so, but it has the hydraulic brake system I mentioned, so it was a no-go for me. Shame, it was a church bus with no evidence of ever been on a school route.
Haven't really seen a bad engine in the Fords, either, apart from the gas 370 having a tendency to burn valves. Usually you'll see 7.3 Navistar, 5.9 Cummins, 8.2 Detroit, and 6.6 New Holland in these, as well as the 370 and 429 gassers. None are particularly bad, but the 370 has been known to have valve issues, and the 6.6L New Holland is a bit tricky to get parts for.
Ditto on the 8.2 you're looking at. I'm not saying don't buy it, I just don't know if I would pay 3500 for it with the caveats of the existing engine. Doesn't really sound like a bad bus, but believe me, you say you don't care about the exhaust stinking now, but wait until you've actually smelled it for long enough. It might be possible to swap a 6.2's injection system to it. I've seen 6.2 fuel system parts used on Oldsmodiesels before.