All you need are a Reciever and Mags
I recently built a kit.
All you need is a reciever, running from about $189 for a Hesse up to over $350 DSA.
AFAIK, all come with the ejector block installed.
To put on a pistol grip, You need 6-7 (6 if no brake, 7 if braked)complaince parts, they run about $100 no matter how you slice it.
Mags are $5 for good condition, $8 new.
It's not exactly easy, but it was not dificult, either.