My $0.02 on this ---
It all depends on what you plan to do with it.
If you're not doing a lot of highway cruising, you don't have a need for speed, and you're not climbing mountains, even a 5.9 Cummins or Navistar DT360 with an AT545 will do fine if not beat on and the transmission does not have pre-existing issues.
Highway cruising, you'd naturally want to step up to an MT643 at least. If mountains are involved, you'll want a bigger engine like the Cummins 8.3, DT466, Caterpillar 3208 (preferably a turbo).
For those swapping transmission only, or engine only, there are also differences in bellhousing pattern to take into consideration in some situations.
And don't forget, the newer the bus, the more involved a swap will be. Most newer buses have electronic controls and controllers that have to talk to each other. Good luck trying to replace a MaxxForce with a DT466...
DailyBee's post makes an interesting point... The only reason to ever buy a rusty bus is for an engine and/or transmission you want to put in another bus. But you really have to consider whether it is worth the cost, effort, and time. And you don't want to pay any more than about $1000 for it.
"CHEESE WAGON" <firstname.lastname@example.org> Suggested reading: (5 threads)