Diesel is right. When I first started I bought a parts D-Boy M4 for cheap and tried to build my own M4 from a broken gun...yeah, mistake. Everything went wrong and I wound up just getting rid of everything. I probably could do it now that I know a little bit more about hop-ups and such, but I would have poured a lot of money into that gun.
My advice is to buy a G&G Combat Machine M4, leave it stock for a while, and then little by little upgrade it if you decide you need to. Then, down the line once you've learned a few things about airsoft guns, and come to love airsoft and feel the need to spend almost all your spare cash on it (as I do >_>), you can build yourself a nasty little M4.