I'm going to spray my standard 07 Vrs wheels gunmetal grey, but what paint should I use ?

Or would it just be easier to send them for powder coating

Ford graphite grey is good anthracite colour, more on the darker side if you want to do it yourself. Downside of this is they chip quite easily.

I'd go for getting them properly sprayed over powder coat.

Powder coating is a super tough finish and really thick to. I'd never have sprayed wheels again after having mine powder coated.