There has been quite a bit of discussion on this thread that goes way beyond just saying Howdy!
I will try and address some of the questions and comments.
Split rim tires have nothing to do with rear gearing outside of the size of the tire on the rim. If the wheel is a Dayton spoke wheel you are limited to a 20" split rim or 22.5" tubeless rim. With spoke wheels the size of the tire is limited to how wide the center spacer is (you don't want side walls to touch) and how much fender space you have. If you have room you could go as tall as 12X22.5. With Budd wheels, if you have the 10-lug hole pattern, you can swap the 20" split rims for 22.5" or 24.5" wheels without any problem. Again, the size of the tire would be limited to how much room you have in the fender. A 10X22.5 tire would be equivalent in size to a 9X20 tire.
I have never heard or seen a medium duty truck rear end have problems that were not maintenance related. If the wrong oil is used, if the oil level is allowed to get low, or if the oil is contaminated the result could be very bad. But speed has never been an issue in regards to rear end issues in my experience. Allison may have some issues with output shaft speeds but that is on the transmission end and not on the rear end.
Getting back to the original posters phases of construction, do the mechancial part first. Or rather, purchase a bus that doesn't need the mechanicals addresses unless you can get the bus for free. You will spend a lot more $$$ and a lot more of your time than you will care to spend if you don't purchase a bus with a good power package at the start.
There are enough buses out there for sale that you shouldn't settle for something just because it is cheap or close by.
While the prospect of driving several thousand miles to pick up a bus and drive it back home can be a bit daunting you may actually save time and $$$ by doing so. The amount of time and $$$ it takes to repair the power package or rust could be more than balanced by going further afield to find a bus with no rust or a better power package.
In regards to preferences for body makers or engine makers most of the opionions are more Ford vs. Chevy vs. Mopar than anything else.
I personally do not like blue Birds and would not choose one if
there was a Thomas, Carpenter, or IC bus with the same power package, condition, and price. But if it had a preferred power package and the price was right I would purchase a Bllue Bird.
I personally do not like Cat engines. In my experience they cost a lot more to operate than the others--they don't call it Caterpillar gold for no reason.
Mercedes-Benz and Ford of Brazil make good engines but getting parts, service, and people who know how to work on them can be few and far between.
The champion in the medium duty truck engine category has to be the IHC DT466. It is a super reliable engine that given even minimal care will see in excess of 350,000 miles. With reasonable care they can go 500,000 miles without any major hiccup.
All things being equal I would purchase an inline engine before a V-type engine just because they pull harder on hills. But then I live in western WA where nothing is flat. If I lived in a different locale where it was comparatively flat it wouldn't be an issue.
At the end of the day, before you purchase any bus you need to make a list of the must haves, must not haves, it would be nice to have, and the is doesn't make a difference haves. The more specific your list becomes the more informed your purchase will be.
Contact me directly if you would like to chat about your alternatives.
Good luck and happy trails.