A G&P body with Colt Canada/Diemaco trades will probably run you $350 alone. Just saying....
As far as scratch building, I wouldn't suggest it unless you have experience in something related like mechanical engineering or are a millwright or something. It'll be a lot of headache without any airsoft gundoc/maintenance experience.
People have easily poured $1200+ into scratch building/modifying a gun to an alright C7/C8 for random things that make it unique like TRI-AD rails and proper trades and such.
Start small and buy a King Arms M4 or something then slowly build up from that changing out parts whenever, it's a lot more cost effective at the end of the day.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE