Get yourself a 16" carbine or mid length. 1/9 or 1/7 twist makes no difference because you'll probably be shooting 55Gr anyways. Get yourself a plain jane M4 "style" carbine. Don't worry about accessories now, just make sure its a flat top and then get a carry handle, or invest in a nice flip up BUIS (to use with the RDS or scope you will buy later) and just practice practice practice. Spend the rest of your money on ammo and magazines. Shoot the gun and then find out what you think will make it better tool for you to use.
Look around at the sponsors up top. Most of them carry AR-15s from the different manufactures and in different styles. Dlask is making some AR-15 uppers in 10.5 if you are looking for a shorty upper. If you want you could get a shorty 10.5 and a longer target 20" later if you want to punch some paper at longer distances.
What do you want to use the gun for?
and if you must, you can change things like the grips, trigger guard and stock when you get it to personalized it but wait for things like a rail and the more expensive stuff unless you just want to spend money and have 5K+ ammo lying around.