i would find out how bad the rust is, remove the door cards and look in the bottom of the doors, if its rusted thru then depending how competent you are you might be able to do it yourself if not then body shop. if its just surface rust sand/grind it back to bare metal then paint it with a rust treatment then paint back over the top with your colour and lacquer. as its bottom of the doors you dont need to worry so much about flatting it off etc. although it depends on whereabouts on the bottom of the doors