I have been using a barn ventilation fan for over 15 years in my shop and I paint on ave 10 vehicles a year. Put the fan close to the floor on the wall away from the door and put enough furnace filters in the door to allow cross flow of air. I've got 4 16"x24" cheap filters. Make a frame so they can be easily removed and replaced. You want to filter the air coming into your booth to help prevent dirt. If your concern is the exhaust particulate then put a down turned duct to a couple inches above a tub of water. The paint will collect on top of the water for clean up and disposal. Just remember to keep water in the tub.
