If you already removed everything from the door, i personally would have them do the whole thing. If they are spraying one part might as well do the whole door.

cost should run you around $200 (if you take off everything: rubber, glass, etc..) so they have no prepping to do whatsoever.

They would have to blast it, probably remove the black, but primer and paint shouldnt cost alot.

Check Maco (sp?). they usually have good deals