There's a guy on ebay selling very raw cocker billet blanks for $45. I've been told they are the same blanks used by CCM. They are raw bricks with the holes tapped - so need alot of milling, not just cosmetic. I think his names dadansonforfun.
All of the halfblock kits I've seen include the appropriately-modified hammer. Future upgrades would certainly be a PITA, probably requiring custom work.
I have one of the Advanced Concepts body kits that are now selling on eBay for around $120. It looks good (black-blue fade) and all cocker parts I've tried to fit have done so without any drama. The Angel-threaded feedneck is a definate plus.
"Skull Paintball" also sells on eBay and theirs are similar to Advanced Concepts, with a bit more mill work. Similar price, too.
The round-top bodies are reputed to be excellent.
I see some turtle Cocker kits on eBay now, hammer included. $350 BIN, but current bids are $1, with 24hrs to go.
Or, you can go with any one of a number of well-known custom shops: Doc's, Martin's machine, Turtle, etc.