Wheelchair equipped buses of that size come in a lot of different sizes even though they might be the same length.
The Type 'A' bus is built on a van/cut-away chassis. The engine is part way under a hood outside and under a dog house inside with the service door beside the driver behind the front axle and it will have a driver's door. The lift can be found at the rear. It is going to be maxxed out for weight long before you every get everything you want installed let alone load people and their junk and plunder. The Ford and GM chassis were about the only choices with the most common engines being the 7.3/6.0/Powerstroke diesel in the Ford and the 6.0L gas V-8 in the GM.
The Type 'B' bus is built on a rail chassis and is commonly referred to a bread box bus because the chassis is basically the same as what is found under bread deliver trucks and UPS vans. The engine is part way under a hood outside and under a dog house inside with the service door behind the front axle and behind the driver. The lift can be found just behind the service door or at the rear. Most were built on the GM/Workhorse P-series chassis and it too is going to max out on weight pretty quickly. The most common engine was the Chevy small block or the 6.2L/6.5L diesel.
The Type 'C' bus is built on medium duty truck chassis. It has the engine outside under a hood with the service door behind the front axle and beside the driver. The lift can be found just behind the service door or at the rear. The bus could have hydraulic or air brakes. It could have rear air suspension. If would be very difficult to put enough weight into a Type 'C' bus and over load it. The most common chassis was the IC/IHC chassis but newer Thomas buses will be on Freightliner chassis and Blue Birds on Volvo/Blue Bird chassis. Cat, Cummins, IC/IHC, Mercedes-Benz, and Brazilian Ford engines were used. The engines of choice would be Cummins or IC/IHC.
The Type 'D' bus is a bus with the engine in front under a dog house, under the floor in the middle, or in the rear. The service door is in front of the front axle and beside the driver. The lift can be located anywhere between the axles or behind the rear axle--it will be located wherever the original purchaser decided was best for their operation and where the engine is located. Engine choices are about the same as the Type 'C' bus. The small Type 'D' buses might have hydraulic brakes but most will have air brakes.
As far as getting the most interior volume for the length of the bus, the Type 'D' is the best. The whole length of the bus will be inside. A Type 'D' with a front engine spe'c'ed with a flat floor will have a flat floor from just behind the front wheel wells to the back of the bus.
Someone just posted purchasing a pretty nice Type 'C' IC bus with the T444E engine of about the size you want. It has a flat floor and rear air ride suspension. Auction hammer price was less than $2,100.00. https://www.publicsurplus.com/sms/au...ew?auc=2048134
Someone else just posted for sale here a small Type 'D' Blue Bird TC2000 with a lift for about $2000.00. And it isn't yellow! http://www.skoolie.net/forums/f14/19...-md-22058.html