You'll find lots of posts on which motor is best. Seems the majority of folks on this forum like International's DT466 and DT466e. International's T444 works for flat land but is underpowered (and harder to work on, if I recall). I was cautioned by the foreman of a bus garage to stay away from the DT365.
CAT motors and Mercedes motors require dealer parts and often dealer repair, so when trouble comes it will be expensive. CAT expensive is less (and likely less frequent) than Mercedes expensive.
Cummins engines are respected, but the 5.9 is a bit underpowered. That's what I have and it did a fine job with an empty bus and flat land. Once I put my junk in the bus and try to climb hills? I'm expecting to arrive at the top... tomorrow. The 8.3 is much preferred (see
one of the many discussions).
Transmissions are nearly all automatics made by Allison. The most common is the AT545, because for the task of picking up kids it works fine and is cheap. But for highway or mountain use, you probably don't want one. Should you refuse a free bus with an AT545? No - unless your in the hills. You can always flip it and trade up! And it may do the job for you. But I would hold out for the AT 643 (it's lockup brother), or the 2000 (the one I have), or the MD3060 (though you may need to beg Allison to unlock the top gear). As mentioned above, you might just find the bus/engine/price you want, and then abandon it if is has the AT545 and buy it if it has anything else.
Others with more experience will chime in, hopefully, and the search bar is your friend. But this is what I've learned. Hope it helps!
