An MT643 is almost worth buying a new bus for! I've never been much of a fan of automatic transmissions in cars and pickups, but I love my 643.
From what I've been observing on this board, you are most likely to find a Cummins 8.3 in a "pusher" bus -- that is, the engine is in the rear. These are also generally the best kind of buses to have -- except that the engine is where you might want to have a big loading ramp. If you can make do with loading your motorcycles and such thru a side door, then you are ready for a pusher.
Yes, the P-7100 is the "famous" inline pump on the 5.9. They are supposedly popular because they are capable of considerable power increase with little effort. Might also be more reliable than the rotary pumps. I do not know what kind of pump an 8.3 would have. And I'm no expert on these things at all!
On my 1992 Blue Bird, I'm not aware of any electrical stuff on the transmission. There is a big honking cable for modulation and/or kickdown. So you may be "safe" at least to 1992.
1994 is when they went to the P-7100 pump on DODGE PICKUPS. Buses got that pump earlier.
If you have been reading on the Dodge Cummins board(s), you have been reading about troublesome lift pumps (which supply fuel to the Injection Pump). If I understand it right, that problem applies to some newer models.
On this board, every once in a while somebody announces they have an 8.3, and we all swoon and drool.