You can use a powder coating as a undercoat and indeed you need to key it before you go further with a paint.
But,.. powder coating is a plastic layer and it is rather thick, IMO it makes the parts have a 'wolly' look.
Go for a primer + 1 or maybe 2 top coat(s) in the color you want.
Anyway, how much use are you going to give it? Powder coating is a good protection for heavy use, a bit much for just the occasional ride. succes.
Hmm, that's two votes against. I'm building this as a 'rider' that I'll keep, but not my regular bike. (Bit creaky now.) At the risk of stirring up an old debate, is POR-15 a worthwhile middle option?