Unfortunately I can not answer either one of your questions directly but I would suggest talking with the local school bus mechanics. I've had tremendous luck with the local school district mechanics. Twice I've had my bus over to the school shop and both times the guys love to check it out and are more than happy to give all sorts of help. Short of them actually doing the work they have been very helpful.
You could also try NAPA auto parts. They deal in medium to heavy duty truck parts so they might have a listing for it in the computer.
As far as my experieince, our 8.3 cummings came with two 8D's. Currently I'm running just a single 8D as one of them gave up the ghost about a year ago. It has been plenty strong enough to start the bus in our moderate northern AZ temperatures.
As far as I'm concerned, you can't use too big of a battery so run whatever you are comfortable heaving into the battery tray.