as far as prep for plastics, I Washed mine down really well with hot soap and water then i them with 320 grip sand paper and then washed them down again. I then hung dried them and used Krylon fusion spry paint to paint them. Worked really well, not having any durability issues at all. It scratches off where my boots rub on it but thats about it. rocks and dirt have not scratched it yet.
As far as the wheels I could sand them down slightly and make sure they are super clean. then use the best heavy duty paint you can find.
as far as colors do what you like!