Talk to Karl norris about the head, he knows lots about 300bhp ZV heads, and frankly most of the rest of us dont, i have no experience of CVH motors at all really, its just not something thats ever interested me, and TBH if i had an rs turbo and wanted 300bhp, i would go zetec turbo instead.
300bhp is a hell of a lot on an 8v head really.

I would be surprised if it comes in at under 5K to be honest when you are finished