EDIT: ^^^^ - What Christopher just said, he just said it more efficiently than I did, and apparently he types faster than me too!
The first question to ask yourself is, Intended purpose.
For me, I plan on using it like its an RV. RV parks, plug in most nights, stop and stay in relatives driveway. Live in it when traveling, but not my fulltime residence.
How many people/animals will be living in here with me.
I will have myself, wife, and youngest son for a few more years. After that, just 2 of us.
Will you be mobile, lots of road time, or light driving and more stationary time.
I plan on about 50-50.
Will you be hanging out away from civilization, driving down forest service roads, and over the top of mountains?
Mine will be a pavement princess, very little dirt in her future.
Will you be staying to one area of the country, or traveling every road and highway you can?
We plan to be all over the North America map.
Towing anything behind?
We will be towing our car on a trailer, or flat towing the Escalade, depends on the purpose of each specific trip.
There are so many other things to consider before you decide which model bus is right to begin with.
If you're going to tow, bigger engine. If you plan on freeway speeds, trans with overdrive. If you plan on boondocking, more roof space for solar array. If you plan on forest service roads, shorter wheel base/more ground clearance is better. If you are hyper hygiene conscious, lots of water and plenty of floor space for a bathroom is a must. The more people and animals you have, the bigger the bus you will want. If you don't have access to a money tree, older mechanical engines are cheaper (not cheap) to fix vs newer computer controlled models.
I had no intention of building a bus, it just landed in my lap. But what landed in my lap is about 90% exactly what I would have been looking for. 40' rear engine 8.3L Cummins, MD3060 trans. The few things I am missing are more head room, an engine retarder, and road A/C. Each of those things are solvable issues with some $$$ and work.
Welcome to the most time consuming, chaotic, rewarding thing you will probably ever do in your life!