I like the bus. I drive Thomas at work.
I like the tranny and all six of those lovely gears!
I know nothing about the engine, but I do agree with anything using early and changing technology is a red flag.
Even if the DPF and EGR systems were not problematic, it's still more to take care of and costly when something does go wrong.
The school mechanics are chasing their tails assuring the DEF additive levels are good.
Look at what happened to the Ford PowerStroke 7.3 (International T444E) when they decided it would be better to stop producing the 7.3 and build the 6.0 when they needed to add EGRs. Then to add insult to injury, they tried to fix the 6.0 issues by making an even worse 6.4 version. Ford finally told International to jump in a lake, built the 6.7, yet even it had a few issues in it's first couple of years. And if you look under the hoods of the same model Ford with a 7.3, 6.0, 6.4 and 6.7, you'll see less and less daylight. ****, the 6.7 doesn't even have fluid change schedules. The computer analyzes the fluids and tells you when you need to have it serviced.
I think the last 7.3 was built in 2002?
Good luck!