Ok, I'll bite since I haven't seen this question a thousand times yet, and I'm fairly new so would like to help out a brother.............that said, I am not an expert!
You will be able to purcahse a complete stock gun from dealers like, GTS, Bravo, Eagle, (check out the equipment exchange site) for probably cheaper than putting together a stock gun yourself.
If you, like I did, want to build your own, you probably will want to upgrade parts to fit your personal needs. things like different handguards, trigger, flash hider, grip, trigger guard, optics etc. No point in paying for something twice.
I ended up spending way more for my weapon than one I could have bought off the shelf, but it is what I wanted. The dealers on this site will be glad to help you out and there are no dumb questions. (I asked all of them!)
You can't go wrong with Bushmaster, RRA, or Colt. Just depends on what you like and what you can afford. For me, I found it was easier as a first mini build, to just get a complete upper and have Bravo Co. install the hand guards and flash hider before shipping my upper to me. I like the 16" barrel and the flash hider does not have to be pinned to make it legal like a 14.5' barrel would. I would recommend an M4 upper with M4 feedramps. The M4 barrel has parts of it machined thinner to reduce the weight vs a heavy all the same (well kind of) thickness barrel.
I'm getting in over my head now, but that's a start............although if you go RRA, get a 1x8 stainless steel barrel imho.....
I bought an LMT upper and put it on a RRA lower with extra goodies I put on myself......enjoy!