Welcome to the site. I went for a long flat nose front engine bus. The reason is that I wanted as much room as possible. I am making a garage area in the rear for compressor, generator etc. The main reason for the garage is to haul my motorcycle. Thus the front engine. Also, it will take up about 8 feet of space.
From the back door to just behind the driver seat is about 35 feet. Take away 8 feet for the garage and I'm down to 27 feet. That's why I wanted a longer bus.
Rear engines are more quiet up front and there is no exhaust or drive shaft running the length of the bus underneath. It would be easier to have under bus or basement storage compartments.
You need to think about what you want to do with the bus before you can determine what to buy. You should also do some engine/transmission research as well as what gear ratio you would like in the rear end. Do you want air brakes or hydraulic? Do you need a bus with a higher roof? The list goes on and on.
The best thing in my opinion is to read as much as you can here and ask lots of questions. Determine what you want and then start looking for it. If you buy the wrong bus and put lots of time and money in it you still won't be happy with it. Hope this helps.