My only experience is painting aluminum Bonzini figurines. I used some automotive paint, which worked very well, and I used plain old Testors enamel, which worked fine but is not quite as glossy as the auto paint. They are both very durable and do not need a clear coat on top of them. The auto paint may have some solvents that could damage the plastic so if you go that route, do a test on a small area first.
To draw on eyes, mouth, etc. I used extra fine Deco color enamel paint pens (red and black).