Here is a list of parts that are needed to complete an AR style rifle. AR15.com has some very good step by step build instructions. It seems overwhelming at first, but once you do it, it's very simple and you have a very good working knowledge of your rifle. Plus, like I stated above, you can use higher end parts for the same price.
Building is addicting. I purchased my first AR. It was a Bushmaster Varminter. Shot great and ran smooth. It had a 24" barrel and I just didn't care for it as it was used for coyote hunting
. I sold it and built the rifle I listed above. I also built a 16" carbine in 5.56 which is now my hunting rifle. I also "built" a .243 WSSM. I purchased the rifle slightly used, tore it down, sent the barrel out to have it cut to 20", reprofiled, and threaded. I then added a different forearm, stock, and trigger (Giessele SSA-E). I also purchased a DPMS AP4 in .308 that I also modified. I had the barrel turned down and I added a carbon fiber handguard to reduce weight. The trigger was modified and I also added a Magpul CTR stock. Then there's the .22 upper that I built from parts...
Build, don't buy complete.
Lower Receiver Parts:
-Stripped Lower Receiver (this is the part that is registered and must be transferred or purchased through a FFL)
-LPK (lower parts kit): This includes the trigger, safety, mag & bolt release, takedown pins
-Stock, buffer tube, buffer spring, and buffer: If you plan on using a collapsible stock, then you need a carbine buffer, buffer spring, and buffer tube along with a castle nut and endplate. This can be purchased together or seperately. If you plan on running a non-collapsible stock (A2, PRS) then you would need a rifle buffer, rifle buffer tube, and rifle spring.
Upper Receiver Parts:
-Upper Receiver: this can be purchased stripped or assembled. If purchasing stripped, you will also need to purchase the forward assist and dust cover assembly.
-BCG (bolt carrier group)
-Barrel: skies the limit with your wallet being the limiting factor. If you really want to go high end, check out Kreiger barrels from Compass Lake Engineering. They have the best prices around and will configure the barrel to your likings.
-Gas Block & Gas Tube: I built mine using a low profile gas block because the forearm would be covering it. Gas tubes come in 3 lengths, carbine, mid-length, and rifle. You just need to match this to your barrel gas system.
-Forearm: Railed, smooth, modular. It's whatever you want. I have both railed and modular and prefer the modular. They are usually lighter, smaller in diameter, and you can add rails wherever you want to add an accessory (light, sight, bipod, etc...)