get the resale thing out of your head, if you dont fix this rust then the acid that is now etched into the paint will proceed to eat all the sheetmetal away and you will then have a giant head ache of a problem to fix which will include welding new pieces of metal sheet.
Remove the battery, neutralize the area with soda scrape off all rust and paint over it, solve the original problem too, why did the battery spill out? did you overfill it?