Best thing to do is go to your local paint supplier and ask them what to use. The only other advice I can offer is be careful if you try to remove decals from a plastic helmet. Ordinarily one would use a heat gun to heat the stickers up so they'll peel right off. Works great on metal or fiberglass surfaces. A buddy of mine did this without checking the helmet type first and melted it with just a small heat gun.
I guess the moral of the story is "don't buy a plastic helmet" at least if you want to paint it.
