That's all minor surface rust. A chassis is made from very thick steel and takes a long time to rust to the point where it's structurally weakened. Your big concern is rust in the body, which is the steel box that is sitting on top of the chassis rails. Your pictures show only the floor of the body, but there is little to no corrosion there. Although the bus is fairly obviously from a part of the country that uses salt on the roads, it looks like the previous owners cleaned the underside from time to time.
You may have rust concerns with the exhaust, which is expensive to replace.