Signal red is a base coat / clearcoat so if originality is important you should probably go that way. But there's no reason you can't use single stage if you can get the paint. I would normally use a 400 to rough up the surface but think 600 would work fine too. There's no substitute for a good retail auto paint store that's willing to give advice. But probably the most important thing is to pick a good paint, get the manufacturer's spec sheet on that paint, and then carefully follow the instructions.