Unless you are really set on building your own, you would be money ahead with a complete AR like the Colt M4 replica made by Walther. I picked one up, and have really been enjoying it. Prob can't find one now, but I only paid around $450 earlier this year...
This helps. I too have been on the fence about a .22 conversion vs. the Colt M4 22. I think Iill wait for a M4 22. Maybe settle for an m&P 15/22