a comment about performance....

ARs in .223 arevery accurate up to 800 yds (in calm skilled hands) but by that range you probly should be shooting anything larger than a P-dog. The Varminter series of AR's are usually 20-24 which increase velocity a bit and will help you get long range accuracy. They also offer a free-float barrel with will also help maintain the accuracy.

http://youtube.com/watch?v=7B9NkQldeu0

Stag is a good company so I expect theirs to be just as good as the Varminters Ive seen. Companies(brands) dont really matter. About the only thing that seperates them is the barrel. With a Varmint rifle you will have to choose between Chrome Molle, Chrome Lined, and Stainless Steel.Most Varmint rifles have heavy SS barrels for increased accuracyand stability. If you dont want to pay that much you can build you ar and try to save some money.

on average the prices wll be as follows (might find lowerif you shop around)
you can get your left handed stag upper and add a 24" SS barrel, and a free float tube.
24" SS barrel should be about 250-300
A3 upper left handed (assembled) is about 190
Left handed Boltassembly(unassembled) 150
Free Float Tube60
Gas Block30
Gas Tube15

talking about 750+ shipping for the upper +
about 270 for the lower

suprisingly the stag price is dead on...
if you build it yourself you get to pay it over time and make it yourself and that wil make it more personal (thats why i build mine) BUT if this is the first time your building anthing like this and if you dont feel your crafty enough to build it using sometimes crappy diagrams then it will prolly be best to just order it from them. It might take a few weeks, big companies sometimes have up to a 12 week wait. But if you order it through them that all parts will fit together best and they garantee MOA where as if you build it yourself from different parts, there is no such garantee.