With that criteria I would look for a Cummins 8.3 if it's an RE and a Navistar DT466E if it's an FE. The Allison 3060 is probably your top choice for trannies in a bus that size, you don't want an Allison AT545. You would be looking for a bus older than an 04, due to problematic EPA regluations that did nothing but add problems and expense to the motor. 37 is only one size smaller than the largest you can get at 40', 14 windows as opposed to 15.
Then a complete gutting of the inside floors, walls and ceiling in order toproperly insulate for where ever you end up.