theres a company here in columbus ohio that builds custom food trucks, carts, and does classic car customs as well... all-a-cart is their name.. if you are looking for a commercial style conversion they may be an option..
the company i had paint my RedByrd can do all the exterior work... remove school lights, stop signs, weld metal, paint, etc.. they specialize in commercial vehicles.. COaches, band busses, school busses, etc.. I havent seen them do full conversions , though they did my and another skoolie exterior recently, and were replacing the floors in a county school bus...
however a couple of the employees are doing coaches of their own to be used for Race-Team busses... so they may be able to do some work for you?
StarJet Motorsports is their name...
I realize you may be far from columnbus Ohio but its a couple ideas..