Great advice to follow but after building and shooting so many I think that i have owned a product from every company in the market. If your wanting a factory built weapon then RRA is the best bang for the buck. Come with the NM trigger, hogue, and they have great customer service, and also they all come with a set of iron sights except the predator and varmint series. Also going with a factory weapon means getting box and paper work. Don t customize and tear your first AR done keep it stock, know it, shoot it, test different loads of ammo.
AS far as twist rate goes then 1/9 is standard and since your first AR then i assume that your not comp shooting w black hills 69 gr anytime soon. Most of your civilian m4 and a4 weapons are chambered in 5.56 NATO and 1/9 twist rate. 1 turn in 9". BC the 55gr and 62 GR are the most common round and can be purchase at almost anywhere that sells ammo and its an effective round. I shoot sub moa at 400 with my 20" 1/9 and 62gr ss109.
Back to your "Build". Before you build an AR-15 remember that everyone has their own preference and opinion and we all like different things. So keep your first one you purchase the way it is and just learn the weapon. Replace parts that are very simple and always keep the factory take-offs. This way you can sample parts that you may want but realize they are not for you. For example free-floating a weapon means taking the FSB off, the FH, and then the Barrel Nut w/Delta assembly. Try a 2 piece rail system so you get the feel of a quad and etc. WHen you are ready to build your first one from scratch it gets almost hard to do but when you decide to build with so many differnt parts from the lower to your Flash-hider to the stock. . . Selling it in the future is almost impossible and you will never get the money back. So your first AR get to know like the back of your hand yet keeping it simple as far as replacing parts. I have found that getting up with someone that has a couple different setups or even parts i am looking at is the way to go before the purchase.
I have a range and maybe one day a few of us can get together and shoot. The first M4 i had was a Bushmaster 16" a# that i changed and changed and eventually went back to my first few changes. I shot it for 3 years before I built my Lifetime AR and 3K in a weapon that is not a 50 bmg or Styer Scout seems stupid. I have killed deer with it,my sister has killed deer at 200 yards, i can shoot 55 or 75gr. Iron or red dot 50 or 400 yards you get my point.
Hope this has helped , just remember that to each his own and you might be a Del-Ton guy where I am a RRA. But remember that you will never go wrong with any product from larue tactical or Lewis Machine: and make sure you are at least getting a Wilson barrel