Unless the buses with the Voith transmissions come with a brand new in the box replacement transmission I would stay away from them like they had the plague. Service, service parts, and shops willing to work on a Voith are basically non-existent.
Don't get me wrong, the Voith is a great heavy duty transmission. But they do get old and worn out. When they go out you are going to be basically SOL and dead by the side of the road.
You can find shops that will work on an Allison in just about every major city in the country and most truck stop shops as well. As far as the Voith is concerned, I might know one shop in the Oregon/Washington area that would tackle a Voith. But the cost will be ridiculous.