You can use a high apeed buffer on plastics just be sure you keep the surface wet and dont over buff or you will go thro tbe paint.
I uswd to do my old car now and again and just did the whole thing,plastics inc, was fine.
I used g3 compound if the paint is flat. or if you just want to shine it up use polishing compound instead, thata less abrasive.
Only do it rarely tho cos you will go thro to the paint if you do it too often.