I work in a paint store
Anyway, the best grit to use to get the original paint off would probably be 100. ONLY use the 100 grit until you see the plastic through the paint. Then step up to 220 or 320 to sand the plastic itself. The finish prep for painting should be 400 grit or higher (the higher the grit, the less sanding you will have to do on the primer)
After that, just follow my paint guide that GTSHYPR posted above!
Good luck, if you have any other questions, feel free to drop me a line!