Hopefully you can put your hands on this before you make a non-refundable deposit. The problem rust would be located mostly behind the tires in the cross members for the body. You may be able to feel soft spots in the same area from the inside.
As far as the mileage be careful since the gauge could have been replaced. Every Blue Bird I have bought from around that year had new gauges at some point. The school system was up front on the replacement and gave corrected mileage based on the ecm. If you don't have a way to check the ecm then maybe contact the school system shop and see if they have any records.
Rear tires look to need replacement.