Hey Anewbus, are you in south Mississippi? I'm assuming there is only one Pearl River Community College which is a few miles from my old hometown.
The Oregon bus looks nice but it's not highway geared and doesn't have air ride. You can see the springs in the pictures that look like non air ride. The PRCC bus probably has air ride front and back and is probably geared better. If you can talk them down any and get the engine fixed it will probably be very nice. Very nice. What the mechanic told you sounds reasonable. And if it's south Mississippi you don't have to worry about rust. Mold maybe but no rust!
Sounds like you're investigating it well. Try to find the shop that maintained the bus and talk to them. I'm trying to think if I have any cousins down there that are diesel mechanics, but I don't remember any.