444 has some great advice. Build your own to learn the system and for the opportunity to build a better rifle than you can buy. If you just want to save money, buy a used AR. Be aware that most people who have problems with "home made AR's" are the guys who bought "kits" and other cheaps parts in an effort to save money.
Here is my advice. For a lower receiver, there are many that will do the job just fine (Stag included) but if you think you will ever sell it, keep in mind that most people consider whatever is stamped on the lower as "What kind of AR" it is. That means a rifle with a Bushmaster lower receiver will generally bring you back more money than the exact same rifle built using a Stag (or other obscure) lower. What ever you do, make sure it's forged and doesn't say Hesse or Vulcan Arms on it!
For the lower parts kit (LPK) - J&T distributing can't be beat. They have one of the best quality LPK's out there for about the cheapest price you will find ($55). The next best IMO is RRA.
IMO, The two most important parts of an AR for function and reliability is the barrel and the bolt. Do not look to save money here.
For barrels, your best bet are Colt, LMT or Bushmaster. They are all 4150 steel and fully chrome lined (well some Colt's have only the chamber chromed). A fully chromed lined Wilson barrel isn't bad either. What ever you get, make sure its chrome lined. Trust me, its not worth saving money here
For the Bolt, Colt is the only one I really trust but BM, RRA and LMT make decent units. For the bolt carrier, make sure it fully shrouds the firing pin. I like a Colt M16 carrier which is fully legal contrary to what some would have you believe. Again, the RRA "Enhanced" carrier is a decent alternative.
For flexibility, use an A3 upper. There is a lot more but that should give you some food for thought. Spend some time on AR15.com and soak in as much as possible before buying anything. Here is an M4gery that I built just to get your mind going
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