Without seeing a pic of the under carriage its difficult to tell the level of rust. If its a little rust it should be fine, but if its all over no, there are better buses out there as people here have said for those prices and that are not rusted. Age doesn't really matter either, I've seen buses from 1960 with no rust on them and some 2012's with too much rust. But there are no buses with zero rust on them if used. They all got it. And you always have to deal and manage the rust, it doesn't go away. Always inspect it. If its a small amount you can deal with it then no reason to not get it.
And if you feel you are not qualified to make that decision bring a mechanic friend with you to inspection. Some people here won't waist time on a bus if they list rust as a selling point and I can see why when there are more choices without that but you might also find a diamond in the rough with a little rust and can manage it.