You don't need to strip or sand the paint off, just make it a little pourus to allow the paint/primer to stick and cure. 800, or even 600 will do it.

OP, are you using a spray can, or a paint gun to do this?

x2 on the body shop if you've never really painted before. A "top notch" state of the art shop is going to charge you a couple hundred bucks, but a local mom and pop's type shop will do it for about $100 and it can/will look good.