I'm not really a fan of Stag Arms, but if it floats your boat, it's your money.
You can purchase a complete lower from just about anybody with a 5 or 6 postion retactable stock. I like the LMT "Defender" lower with the SOPMOD stock. It's the most popular with the "GO FAST" guy's.
Choose any higher quality upper and pin it to that lower and you'll have hell of nice rifle.
Things that are non-negotiable for my AR15's.
1. 4150, Cold hammer forged or stainless barrel.
2. 1/7 twist rate. This allows for heavier loads to stablilize.
3. M4 barrel extension.
4. M4 cut feed ramps.
5. HPI tested
6. MPI tested
7. Mil Spec dimentions on both recievers and extension tube.
8. Only quality high end rail systems. LaRue, Daniel Defense, Knights.
9. Chrome lined or Nicor treated barrel.
10 Low profile gas block with rail system.
11. Front and rear flip up iron sights. You could use a front sight block with integral flip up sight but I like my iron on the rail. Troy is my choice for both front and rear sights.
12.T-marked Flat top upper reciever. This is common, but it's worth mentioning anyway.
13 5.56 chambered barrel. If you purchase only the .223 then you are limited to only .223 dimenions and pressures. The barrel is generally marked as Nato 5.56.
14. Mil Spec anodized parts, at a minimum. Some finishes are better, but more expensive. IE: Les Baer Bear Coat.
15 SOCOM extractor upgrade kit. This only cost $3.99 and is another non-negotiable item in my bolt.
I'll think of more in a bit.