If you sand too far and go down to the metal anywhere, you might want to use a metal etch primer in those areas. Then I would use a single stage eurethane. If your not changing the color...I see no reason to prime it. I'm sure that others will have opinions as well.
I would use a metal etch primer as I said before, then apply a single stage eurethane of your color choosing over the top and be done...Or you can shoot a base/clear if that's your preference. The amount of paint depends upon the gun used, air pressure, number of coats, etc, etc. I would rather buy too much paint than not enough....This is obviously for the metal surfaces.
Are you painting any fiberglass or plastic cowlings/parts?