Low-buck S-system build up

[machgo]

I have a project idea to build up a low cost A&K M4 S-system into a metal body gun with greatly improved performance. It seems the metal body upgrade is tricky with the S-System. From what I gather on the web, either a Zeke body, or a Hurricane adaptor set, is needed. The Zeke body is hard to find, and the cost of the adaptor AND a metal body is a bit of a budget buster. Also, I've read that the quality of the Zeke body is iffy. Can the plastic upper of the original gun be fit into/onto a metal lower receiver? Please chime in with any suggestions! Thanks in advance