Get a CA M15A4
Basically an M4, the CA will be full metal, and CA's quality quality control(although still not equal to TM's) has gotten much better over the past feew years. The CA
might be more expensive, but by the time you add a metal body to the TM, and bring its internals up to the CAs you probably wont have saved yourself anything.
It's better to light a candle, than to curse the darkness