Greetings, and welcome!
I'm no mechanic so am not an expert on things, but from what I've picked up here that MT643 tranny is a good one. I think I've heard mixed reviews about the cat 3208. That is not a lot of mileage for a bus, but you'll want to verify that as it's a 22 year old bus!!! I'm sure someone more knowledgeable than I will be along shortly to tell you more.
One source for buses in Washington State is government surplus out of Auburn. They sell a lot of vehicles and I bought my bus there. here is a sample of vehicles for sale right now:
International/Bluebird 1993 Bus
DT360/MT643
Wheelchair Lift
http://cgi.ebay.com/ebaymotors/Inter...2986QQptZBuses
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Ford 1987 Bus
Larger van/nose style bus
V8 Diesel, blown engine
Wheelchair Lift
http://cgi.ebay.com/ebaymotors/Ford-...4625QQptZBuses
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Chevrolet 1981 Stepvan Bookmobile
350 Gas Motor
New rear rims and tires, carburetor and transmission
430k miles
http://cgi.ebay.com/ebaymotors/Chevr...rcialQ5fTrucks
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Granted, their not all buses and they don't all run, but it gives you an idea of the variety of vehicles they sell. Sometimes they'll list a dozen buses at once, sometimes weeks go by without any.
Hope this helps!
-Ray