Most desirable power trains are international dt466 and cummins 6cta 8.3 (both mechanical older engines), pre 2004 international dt466e and cummins isc (computer controlled without egr valve). These are wet cylinder and can be rebuilt inframe.
International t444, t444e and Cummins 6bt 5.9 , ISB are ok but a little light on power. CAT 3126 are ok but parts are more $$$. All above are parent bore and have to removed from the bus and machined to rebuild. Stay away from engines post 2004 as they have emission systems which are more $$$ to fix.
Good trannies are allison 643, 2000 and 3000 series. Allison 545 lack a lockup torque converter and are not ideal for highway use due to overheating and lack of engine breaking going down hill.
Ted