Another consideration would be traffic flow on the site. As long as you have adequate stacking (entrance) and turn out (exit) at the bay, you could easily put a
PDQ M5 with 26' rails (be able to wash the duallys / crew cab PU's in your area) in this bay with an outside dryer mount and still have room in the bay for
doors. There would be no operational need to modify the bay.
Dryers mounted on the outside of the bay exit can look / work fine. As far as on board vs drive thru, it depends on the area. An on-board removes the customer element (speed) on the dry, but reduces thruput. Drive thru with a timer can deliver the same quality, but is more dependent on the customer and how they use it.