Rust issues are difficult to fully diagnose when seats and flooring are still installed.
If you can inspect from underneath, a thorough inspection of the floor and supporting structural steel is about your only way to inspect the bus as it sits on the lot.
In my opinion, for all it's worth, you'd do better to look at busses away from the nation's rust belt.
Good luck, and post pics of it if you can. That might get you better advice on what you're looking at...