The powerstroke 7.3 is a highly desirable engine. Some might buy the bus just for the engine.
The last year the 7.3 was made was 2004, so it might have some emissions on it. They stopped making the 7.3 because of the emission requirements.
The cutaway van buses are generally well geared for highway speeds and the 7.3 will definitely pull that size bus up a grade.
The 7.3 came as a powerstroke for Ford with more HP and less torque, and as an International T444E, more torque, less HP for commercial vehicles. The main engine is the same, but a few major components, tuning, ecm, etc. are different. My bus has the T444E and it's a 33', so your engine in that size bus is easy peasy for traveling.
As Cheese said, a well cared for diesel, especially the 7.3, can go a long time. Fluids and filters routinely is the key. If you have records showing that, I'd consider it.
The other thing to remember is it's not just the engine that's gone 300k+, it's the tranny, suspension, brakes, etc.. In general, those parts are not designed for that kind of mileage, so if they haven't been cared for, recently replaced/updated, etc., I'd be concerned about that.
If you can get it fully checked over, engine, tranny, suspension, brakes, etc., before buying, it would be well worth your money.
Best of luck.