Regarding drive train choices:
The MCI will likely have a Detroit 8v71 or 6v92 or possibly an 8v92. If it has an automatic transmission it is likely an Allison HT-740. All good. I put 250k miles on a bus with the 8v71 & HT-740. It served me well and is still on the road with it's new owner.
As far as skoolies go, I assume that you are looking at a full size bus. If so, I would definitely stick with the 8.3 Cummins. I have owned several 5.9's and love them in a pickup. In a big bus the 8.3 is a better fit.
That's my $0.02