AR15.com has an awesome retro forum. AR15sport.com has many of the parts needed to do an A1 rifle. Those kits mentioned above are definately no bargain.
I used a Dimeco C7 upper (i am left handed) a new barrel 1-12 twistand bcg, new reciever and old parts also from AR15sport. My rifle is awesome and shoots 55gr bullets lights out!