A few pointers to keep you out of trouble...
I will respectfully disagree with the suggestion to buy a preban...
Don't waste the extra money just to poke holes in paper...
They look cool, and hold their value well, but the telescopic stock is just plain uncomfortable to shoot...
Period...
For a 1st gun, I'd strongly suggest a brand new, or like new FACTORY built rifle from one of the better known companies...
(Bushmaster, Colt, RRA, etc...)
Stay FAR away from fancy overengineered carbon fiber guns, and frankenguns assembled privately on a stripped receiver...
There are too many good factory guns out there, to bother risking trouble with a home built gun...
I've built plenty of them from scratch, but not until I knew them inside and out, and experienced every possible problem associated with the AR15...
The best AR I've ever owned was a brand new Bushmaster XM15E2S 20" HBAR target rifle...
The only brand new AR I ever bought...
This gun shot great right out of the box, and never gave me a bit of trouble...
It's totally up to you whether you prefer the 16" or 20" barrel length...
I like both, but for different reasons...
The 16" is short, lightweight, and handy...
The 20" is a bit more accurate, and recoils less due to it's heavier weight/longer barrel...
You should be able to find a used AR in the Equipment Exchange for around $650-$700, and a brand new one for $750-$800...
Also other things to be aware of...
1) Use only good quality USGI mags...
2) Use only good quality military 5.56 ammo...(Winchester Q3131A)
3) Keep the gun CLEAN...
4) Keep the gun well lubricated with CLP "Break Free"...
5) Stay away from muzzle brakes, all they do is piss off the guy next to you at the range...
Hope that helps...
ART