Pip, its pretty easy.
step 1
get a quality Mil-Spec complete lower (dont have to install the LPK). LMT, Sabre Defence and Spikes Tactical seem to be the big favorites. Get the grip and stock you wish (I prefer the Magpul MIAD for grip and the CTR or UBR for the stock).
Step 2
Get a High quality upper. BCM, Noveske, LMT, Colt are the most common as far as quality, and the new Charles Daly DM4-LE is getting some great reviews. Examine the BCG to make its properly staked, if not send it off to a gunsmith to have it properly staked (or buy a MOACKS that does it).
Step 3
Put them together.
You will pay more but you get what you pay for. Stag, RRA etc. are all fine and dandy for plinking and general range time but I am building SHTF guns so the standard I will hold my parts to is higher.
For example is the Bolt Carrier Group. Stag, RRA, Larue etc make some good products but they batch test on the Magnetic Particle Inspection and High Pressure Test on the bolts, barrels etc. When you buy a BCM, Colt, LMT these parts are individually tested and as such the price goes up.
M4Carbine.net has put up what they call
The Chart and look there to help you make some decisions.
I am currently progressing my build in the Build it Yourself section of the website. I plan to go step by step showing before and after photos of two ARs. I hope to give feedback on the parts I used as well as the place I have them.
Between Stag and S&W I would get the S&W hands down. I would just grab a Mil-Spec barrel and be happy.
If you have ANY questions about it, feel free to PM me.