For what its worth, though you can buy one as cheap as build, I applaud you for doing so. I assembled both of mine, and they are exactly the way I want them, not the marketing guys or the guys in accounting. As such, they are MINE, and may not be suited for others, but I didn't build them for someone else.
Anyway, here is a thought, since you are starting from scratch. Unless you have a Dillon 650 or an uncle who works at Federal or Hornady, why not find something cheaper than the. 223 to shoot? Since this is going to be a zombie apocalypse gun, why not shoot it cheap(er)? You can get a barreled upper in 5.45x39 for a little more than a standard 5.56 upper, but the upside is that the ammo is about 12 cents a round vs. 35 for new FMJ 5.56. It uses slightly different magazines, and there is quality ammo available for when you want to drive tacks.
Just a thought, but that is going to be my next build. I can hoard several thousand rounds of ammo for under $500.