Since you plan on doing a lot of miles, make sure whatever you purchase is up to the task of traveling at highway speeds.
There is nothing wrong with a bus that has a top speed of 47 MPH traveling across our country in the slow lane or on back roads.
But if your plan includes long stretches on the Interstate system you need to find a bus that has the HP and gearing to travel at highway speed.
I would also tend to agree that 15' is not a lot of space. You say you have lived in a camper and in a truck's sleeper. Those can be very tiny spaces. But how often were you in those small spaces with two other full size adults? I owned an 11.5' cabover camper. Any time there was more than one other person in it I felt cramped. In fact, my step-father always referred to it as the cramper.
If you can, you may want to lay out on a garage floor or back yard the floor plan you want. Use cardboard boxes to use as walls and garden furniture for your table and chairs. After laying it out you may want to add 5' or 10' or 20' to the length.
The greatest amount of fuel is used in poking a hole through the air. Adding 10' onto the back will not use appreciably more fuel.
Over the years I have come across several really nice lift equipped buses in the 6-8 window length that had the same power package as the 12-14 window full size buses the school used. The heavy duty package tended to ride a little rough but the big engine and transmission meant they went forever with little or no repairs.