Keep a close eye on the PSA(Palmetto State Armory)website. They run specials that have the complete gun or the upper and lower seperate. Sometimes the lower is "complete" (with BCG and charging handle),sometimes not.
For your first weapon,at the cheap prices nowadays . . . BUY. I have seen a complete AR/308 for <$600. This would be a good place to start your build.You could slowly add this,and change that,until you had a gun that was "yours".
Most AR/308 parts are the same as any other AR parts. Sign up for the internet flyers. From PSA,Cheaper Than Dirt,Kentucky Gun Co.,and others. The specials come and go pretty fast. Scan these sites often. --- pruhdlr