Many people did it already, on the Antec P182 for example, to get a black interieur. My first bet would be maybe some sanding, then normal primer, then normal spray paint. The same way you would paint the exterieur of the case while on a budget.
Powder coating would be the more durable though more expensive alternative.
I did it with my Thermaltake Shark by doing some light sanding to remove the waxy/protective coat and then spraying on some bare-metal paint. I can't recall the brand, but it's matte and gritty. It great and was surprisingly scratch resistant with no baking.
Look in the paint aisle for bare-metal paint. It's usually used on outdoor metal furniture. Just don't spray it on thick. Several very, very thin coats should be more than enough to cover. I'd recommend matte colors over gloss, but that's more of a personal choice.