Krylon makes spray paint for plastic now. Would still recommend using an automotive primer first, then Krylon paint, then some sort of clear coat to protect the paint.
Looking forward to the progress! I built one of these for my son for the TFA premiere last year. Still have some adjustments to make, and am currently modding his Adult Rubies 2 piece helmet to have a real lens.
I did not repaint my boys last year, but am highly considering doing so for the upcoming Convention season.