A very good AR specific forum
M4carbine.net is an excellent resource. There are lots of serious AR shooters and industry professionals there. TFL is my favorite firearm forum, but for more specific info, M4C is excellent.
As to the pros/cons of different setups, my recommendation is for your first AR, buy a complete, basic, high quality rifle. I have a Colt 6920 that I've "fixed up" with Magpul stock, grip, and forend. I put a Burris 332 (3xilluminated reticle) on it and have been very happy with it. Now that I know what I want, I'm going to build one with all the options that I've decided I want.
The most important accessory for a first timer to buy is ammo and range time. Learn to shoot it, learn to perform immediate action drills, field strip it, clean it, hug it, pet it, name it George, whatever. Learn it inside and out, THEN figure out what you want and build one if you want.
Having said that, I bet my 6920 and I will outshoot and outlast anybody's fancy rifle of awesomeness. One thing I learned at Sand Hill is that the basic AR is an excellent piece of equipment. It's all in the rifleman.
In conclusion, practice, practice, field strip, clean it, then practice some more. Just about any manufacturer's rifle will work for what you need in the beginning.
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