My 2004 6.0L E-450 has 158k miles. I was told by the guy who owns DP Tuner that the bus engines are very detuned. Which might mean they'd be in better shape than the engine in a truck. Mine appears to be in very good shape, as far as I can tell. None of the telltale signs of impending failure.
So I tore into it proactively, to avoid future problems. I removed the everything in front of the engine (radiator, intercooler, fan, etc), everything on top of the engine, and the exhaust off the rear of it. New belt, water pump, PS pump, new bearings in the idler pulleys. New oil cooler, TIG welded the EGR cooler's exhaust passages and reinstalled it. Stiffer blue spring in the fuel pressure regulator. Cleaned out the turbo vanes and got them working again. Also had cold start / rough running problems and it turned out the be a bad power supply board in the Fuel Injector Control Module. Dorman replacement is $93 on Amazon and simple to change. I pulled mine out and re-soldered per instructions I found online. Truck starts and runs fine now.
I learned how to do this stuff by watching videos on youtube. Search for it.