Re: Does this combo exist?
72 passenger rear-engine school buses are usually 36'. Some REs were ordered as 13-row bodies with 12 rows of seats (mine is like this), so you get a 38' body. They were available down to 60 passenger, which would be closer to 32-33 feet.
City transit buses are/were available from most manufacturers going back to the 50s and 60s in 30, 35, and 40 ft lengths.
Headroom is usually 73-74" standard, 77-78" optional.
Lift doors were usually placed in the middle to keep the wheelchair positions near the middle. Wheelchair passengers need a clear path to the emergency door that is wider than a standard aisle, so they tried to keep them as close to the emergency doors as possible.
Many REs have additional emergency exits on the right side, which are usually located slightly to the rear (and narrower than a lift door). However, most are still positioned in front of the rear axle. The specific design you want might have been ordered at some point, but it would be very rare.
My advice: Find a good RE with everything else you want, and add the door where you need it.