First, head over to Ar15.com and check out their "build it yourself" forum. LOTS of great info and advice for people in your situation.
For me, I spent a few months reading over there before I decided my course of action. I settled on assembling the lower receiver myself and ordering a complete upper. Once the two halves are complete, it's just a matter of putting them together.
Here's some questions for you:
Are you a DIY/handyman?
Do you own pliers, screwdrivers, or a set of punches?
Have you ever detail stripped a weapon?
Do you follow instructions well?
Do you have the patience to go slow and do things right?
I'd say if you answered yes to any two of the above questions you could build your own lower receiver with your choice of stripped lower and any quality (Stag, CMT, RRA, DPMS, Bushmaster, Etc) lower parts kit.
From there, just look online at the different uppers available. Things do decide on are barrel profile and length, gas system length, sight radius, upper receiver, hand guards, et al. Don't be intimidated by the options. Just think of it as a really cool Lego set. One of the most important parts of the upper is the bolt assembly. Typically, if you have a failure related to your upper, the bolt caused it.
The next decision is even more important. Ask your self, "what am I going to use this for and what kind of quality do I need?" This is pretty much the same as "how much do you want to spend?"
If you purchase say a Del-Ton Inc rifle upper. You can look to spend in the neighborhood of $400. You will have a very nice hobby rifle that will be great for taking out to the range and plinking around with. MANY, people love Del-Ton for their low cost and quality parts.
Lets say you move up the quality scale to a CMMG upper. You can expect it to be closer to Mil-spec and a higher quality with tighter tolerances. However, you will also see a price increase. Look to spend around $650. This would be a great compromise for price and quality. This rifle would likely make it through carbine courses with little issue if well maintained.
Another hop up the quality ladder puts us at "Tier 1" as the hardcore AR guys like to refer to them as. Brands like Colt, LMT, and Noveske. These guns tend to be practically mil-spec with Colt's being the "real deal." Look to spend $800+. These rifle will be suitable for duty work and anything else you want them to do. Many are convinced you build with 100% "Tier 1" parts or you buy them new.
Hope this gives you a little insight. I completed mine on 9/11 and have been completely satisfied with it. I used a DoubleStar lower, DPMS LPK with a RRA trigger, and a Stag M4 upper.
Later,
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