Beautiful bus! Coaches like this were meant to go forever, as long as they are maintained. You really want to have a good look at the frame, especially around the rear and front wheels. Look at the supports that hold up the body. If they are in bad shape, it's probably done, spare parts for another bus. If they are solid, that's one point on it's side.
Someone else will know about the engine, I am a dolt when it comes to engines.