I can't remember if they're clearcoated, but i would assume they are. If that's the case, just scuff them with something (Scotchbrite scuffpads [gray pad]) would be the best. When you scuff, scuff real well to make sure the shiny is gone especially in corners and cracks. Don't go too hard on high edges that you break through the clear.
I wouldn't use paint thinner, start with a good degreaser, soap and water wash, then scuff, then alcohol, then shoot 'em. You dont want wax or any other silicone based materials on the wheel when your painting (that includes wiping them down with a rag you've ever used for waxing)
I've yet to see a powdercoat job that i really like. We do alot of painted wheels for customers, and have had a few sets powdercoated from different places, and ive always liked the painted more. Satin semigloss black is the only color that seems to look better powdercoated.
if they're not clearcoatd and are bare aluminum... painting them would be a pain, they'd need a real nice epoxy prime to adhere well. I wouldn't recommend doing it yourself, and it wouldn't be very cheap to have done. Buy a foose powerball, polish 'em real nice, sell 'em and then buy yourself paintable wheels.
good luck!
too much info?