$3,500 is too much $$ if you ask me. Unfortunately for your friend, skoolies don't have much resale value.
I bought a 1991 ford/Ward 72 passenger with 250K miles on it for $775
My "new" bus is a 1991 International 72 pass with a much sought after DT series motor with 180K on it and i paid $1,500 for it.
To put it mildly, the 6.9 liter with the auto trans is gonna be a dog, but who cares? You're in a bus, not in a hurry.
I think $1,000-$1,200 is a very fair price for that bus. I believe the 6.9 liter is fords pickup truck motor (made my international, but it's not not to be confused with the 6 cylinder DT series motors), the precurser to the 7.3 liter. The 6.9 is a non-turbo mechanicaly injected V8. I don't know what the life expantancy is of that motor in the skoolie application, but I would venture to say it's not a 200K mile engine.
Quis custodiet ipsos custodes (who will watch the watchmen?)