Seal the inside walls with caulking when you panel or sheetrock it to eliminate drafts. Your exhaust fan should have ductwork along the floor so that dust and overspray are sucked down to the floor and then out. Use Furnace filters on the air intake. You need incoming air for the exhaust fan to work properly. Spray the floor with a hose before you start. You could spray down the walls too If you don't accumulate a lot of clutter in there. When I was painting in my dusty garage I stapled a plastic dropcloth to the rafters over the car to catch any dust that might fall.
AL