School buses come in four different types with variations within each type.
Type 'A' come in single rear wheel and dual rear wheel. They are the small buses and are usually built on a cut-away chassis. Most come from the factory with a lot more bus that there is truck underneath. If you choose to go this route make sure you weigh your gutted bus and then weight everything you put in so you don't go over weight.
Type 'B' buses are known as bread box buses because they are pretty boxy and a long time ago they were built on IHC Metro or GM Step Van platforms. The buses have a hood and the service door is behind the front axle and usually behind the driver. These buses are also usually a lot more bus than truck underneath.
Type 'C' buses are also known as conventionals. They are the most common bus on the road. The service door is behind the front axle and beside the driver. Common usually also means least expensive on the used market. They come in lengths any where from as few as four rows up to thirteen rows of seats.
Type 'D' buses are also known as transits. The service door is in front of the front axle and beside the driver. They come with the engine in the front (FE), rear (RE), and there are a few with mid-mount engines (Crown and Gillig). They come as short as six rows and as long as sixteen rows. Some of the really long ones come with a third axle.
There is no one bus that is perfect for every application which is why there are so many different types available.
For your purposes you need to decide which type of bus you prefer. The advantage of the Type 'D' bus is the whole length of the bus can be made into living space. The real disadvantage of the Type 'D' bus is since the engine is somewhere "inside" the bus body you have to deal with noise and heat from the bus while going down the road. The real advantage of the Type 'C' is they are built on a conventional medium duty truck chassis. Parts and pieces are available for them at just about any parts house in the country. The real disadvantage is you have 4'-8' of the bus sticking out in front of the windshield. The real advantage of the Type 'A' buses is their size which is also their biggest disadvantage. I am currently driving a brand new Type 'A' bus that I am taking out to show to customers. It is a white MFSAB so it doesn't have any of the yellow bus equipment on it. Even still, it can only carry an additional 4K lbs. Most Type 'C' buses have a carrying capacity that is well in excess of 6K lbs., some as high as 12K lbs. The higher carrying capacity translates into transmissions, brakes, axles, and tires that are much larger which results in fewer repairs.
If you really want a shorter bus you are more apt to find a Type 'C' of the size you want than any other size.
I personally do not recommend anybody try to convert a Type 'A' or 'B' bus because there is so little truck under the bus. Besides, you can usually find a Type 'C' bus for less $$$.
If I was going to convert a bus this bus would be high on my list of buses to consider. 1990 International BB 1HVBB27N1LH288586 - TheBusDealer.com - Auburn, Washington 98001
Not only does it have great rubber (no recaps in the rear and at least 80% in all positions) but it has highway gearing and a 5-speed automatic with a deep low gear to get you going on a really steep grade. It is also a 10-row bus so it isn't full length.
If I was looking to purchase a Type 'D' that wasn't that long I would seriously consider this bus. 1998 BLUEBIRD RE RE 1BAGGB7A4WF083842 - TheBusDealer.com - Auburn, Washington 98001
It has the Cummins 8.3L so it will have lots of go when out on the highway. It has two service doors with a lift in the rear door. Which will allow you to decide if you want a front door or a door in the middle of the bus or two doors.
We have some really nice 8-row buses coming in on trade in the next few weeks. They will be priced below $6K. They will have less than 150K miles.
Again, before you purchase you need to decide what type of bus it is you really want. Once you determine type then you have to decide which power package you can live with and which ones you need to avoid.
Good luck and happy trails.