Commercial paint booths are built with what's called laminar flow down the length of the booth. Enough air volume pumped through so there's a nice gentle flow down the booth length. Your concern about the fan motor not being rated as explosion-proof is valid. If you had a fan with an explosion-proof rating, then I'd say use it to exhaust air from the booth with a wall of filters on the intake side, and good door seals to keep stray air from leaking in and potentially stirring up dust. Yes, dampening the floor is to keep dust down.
FWIW, I painted my truck inside a tent, with little dust protection other than wetting the floor underneath. I had very little issue with dust nibs on the finish.
