Doing the firewall and the fenders is a bit much. You might consider haveing the paint put in spray cans. There is a company that will do that for you and its not that much money. Also, consider useing a sprayer from this company.
http://www.prevalspraygun.com/ They sell them at most major hardware stores. One more option. Use an undercoating for the fenders. You can get that from Eastmans. Cecil.....