Probably don't need an exhaust fan unless you plan on running the vehicles inside, or you are doing metal work and grinding, or wood working. If that is the case you could go with more of a dust collecter, for wood and metal working. Any air that is sucked out by an exhaust fan will need to be replaced or made up with fresh air, almost always air from out side, so depending on the size of the fan it could cool down the garage if it's cold outside.
If I'm wrong please let me know!
IMO my garage gets dirty or dusty from having the door open and wind blowing stuff in.
I would suggest a very light gray color for the walls. That way if you do happen to get a dirty hand print on the wall, it's a little easier to hide. At work we have white metal walls and always look dirty, even after we clean. Just having a lighter color on the walls will really help brighten up a garage. Not sure if you need hi-gloss, unless your thinking for clean ups.
Not trying to convince one way or the other, just my opinion.