RE: to build or buy whole?
Cheaper to buy a whole gun.
Buying a barrelled receiver without stock or forearm will cost nearly as much as the entire rifle. If you only want to change out the forearn or buttstock, buying the entire rifle is still a smarter move.
IMO the only time buying any rifle as different parts and putting them together yourself makes any sense is when you have a strong desire for a highly customized rifle.