Rust is sort of an eye-of-the-beholder kind of evaluation.
Bad rust as observed by someone from the dry SW where paint bubbling is considered bad rust is not the same as bad rust as observed by someone in MI.
I purchased a used bus in Grand Rapids. I was surprised at how little rust was under the bus. I have actually seen more rust on buses that have seen all of their service life in WA state.
Since you live in MI you should be able to evaluate how bad the rust might be on that bus.
Even if you were able to purchase a bus for less $$$ somewhere else you will need to factor in how much it will cost to get it back home to where you are--travel to where the bus is, meals and lodging there and back, fuel, and the real possibility of a break down.
Personally I think the bus is a pretty good deal as long as rust isn't an issue.