Wow. Lot of opinions here.
I like the idea of using a Nickel Boron (Also known as EXO or Fail Zero) bolt carrier group (BCG) and fire control group (FCG): Why? Durable as hell and super low maintenance. You need something that's going to be forgiving about cleaning when the SHTF.
I'd recommend also the 16" barrel, although whether you like the pencil barrel, government/M4, or HBAR is entirely up to you. The pencil barrel is the least awful to carry distances, the HBAR is the most pleasant to shoot.
For sheer maneuverability at the 16" barrel length, the lighter the barrel, the faster it moves. Period.
I have to recommend AGAINST a "mid length" gas system as it is considered NON STANDARD. In a SHTF situation, your parts supplies are going to be limited. Going with something oddball, that is NOT currently in the Mil Spec, means you could very seriously single-source yourself into a situation where you have no resources for spare parts. A standard carbine upper with a carbine length gas system -- you can get the gas tubes pretty much anywhere, and drop the part in with a punch and some patience.
If you were planning on using oddball ammunition like heavy bullets, in say the 69 to 72 grain range, then I'd recommend the 1:7 twist. But seriously, for an all purpose rifle, I'd recommend going the same way the military did: 1:12 was too slow, 1:7 was too fast, 1:9 was Just Right.
Further, I'm going to recommend you NOT build your first AR. Buy a quality gun so you have it for defensive purposes. After you figure out all the ins and outs of this particular platform, only then should you attempt your own build.
Those are my recommendations.