Those pics honestly look like the exhaust manifold, which isn't going to hurt anything, it looks like typical manifold rust. Not a big deal because the manifold is thick cast iron and doesn't affect structural integrity anyway. Exhaust being rusted is a foregone conclusion, it is designed to be replaceable.
You need some shots of the flooring from underneath the bus and frame rails front to rear -- THAT will tell the tale... You also need to keep in mind that a 34-year-old bus is going to have some rust unless it spent all its life in TX, AZ or something like that.
Where is this bus currently, and where was it used when in service?
"Cheese Wagon" <firstname.lastname@example.org>
Former owner - 1989 Ford B700 64-pass Blue Bird (Rest In Peace, Cheese Wagon)