Its a loaded question LOL! People have used a lot of different paints with success. Automotive paints, marine paints, epoxies etc. I have decided to go with plain old tremclad rust paint
. Others have used it with good results. I am going to use a 4" foam roller to apply. Tremclad is reasonably priced and readily available. Anything will be better than what the PO brushed on it. I think it comes down to how much you are willing to do yourself and how fussy you are. Do the best you can yourself or take it to a body shop.