IC buses went to 12" windows as standard across the board starting in about 2003. All the rest still have 9" windows as standard.
It is obvious when looking at pictures which buses have the 12" and which ones have the 9"--the top of the 9" windows line up with the top of the driver's window. Even still, the inside height is still only going to be 78" or 79".
Generally speaking, RE buses with luggage compartments were spe'c'ed as trip buses. Generally speaking they will have the higher headroom, bigger HP, and highway gearing that you want.
All school buses with the exception of Crown and Gillig mid-mount engine buses have no problem finding a place to install a hitch receiver since the rear bumper is usually bolted to the ends of the frame rails. The Crown and Gillig buses that have mid-engines have very little in the rear trunk that will support the weight of pulling a trailer or the hitch weight.
For your purposes a 5.9/ISB is not going to be enough engine for you. Yes the 5.9/ISB can have upgrades similar to what can be done to the same basic engine in a Dodge pick up. The problem in a bus is the cooling system and the transmission will not be up to the task. And upgrading the cooling system to handle more HP in a rear engine is a big deal.
Probably the best power package would be a DT530/Allison 3000. IHC had that engine as an option from about 1996 to about 2004. 310 HP is always better than 285!
Good luck and happy trails!