Welcome to the site.
As mentioned, rust. Then a good drivetrain inspection.
If it was maintained as it should have been, 80K is relatively low mileage for that bus.
As is with all significant purchases it is always buyer beware. If you don't know what to look and listen for on your inspection of the bus, try to find someone who knows what to look for before laying out any $$$ on something that you can't fix.
A 4 window is small inside for two. Hopefully you are good at being close for long periods of time!
Good luck, and post pics if you go forward with the bus...