M4carbine.net is a much better source but the focus is on combat capable rifles. Firearms Training and Tactics Network is another good one that a couple guys started, one or two from here, if I'm or mistaken. Then there's Lightfighter and a couple other semi-closed forums, but tread very carefully as it's very alpha-male, many with good reason though.
As for the AR itself..... Bravo Company is one of my first choices. They have pretty much everything and anything you'll need. Centurion Arms is great and also an awesome choice. Owned by Monty LeClaire, an active duty SEAL and one of the nly sources for a mk12 with the issued barrel from Douglas turned down by the same people as the issued barrel. Then there's always the Colt 6920 which is an awesome rifle and the standard for hard use. Spikes?? well, they work. Daniel Defense will always be a good choice, especially their rails which I'm using now in place of my KAC's. Knight's Armament... Expensive and proprietary and needs a $250 wrench for to remove the URX II or III rail. I had two. I sold two. You get a whole lot for your money which I've broken down in detail before. Personally I've decided to go back to the standard components since I break so much stuff. Palmetto State is good too, but there are growing pains. LMT is always a good choice, just keep in mind they do not offer a mid length in their standard offerings, nor does Colt.
Bushmaster, DPMS, RRA, Olympic Arms, Stag.... They can work. If your rifle is for defense or work and you do not already own one there's no need to start now. Stick with one in the above paragraph with standard parts. I'd recommend a BCM or Centurion 16" mid length gas system instead of carbine, though a carbine gas system will work fine. Stick with a flash hider instead of a brake, at least for now. GI trigger then upgrade if needed. It'll come with the auto carrier and H buffer. Upgrade parts only when necessary. Get a good sling and light, preferably from Surefire. Quality irons from KAC, Troy, LMT or Daniel Defense are good. An optic from a reputable quality manufacturer like Aimpoint, Trijicon, EoTech, Elcan, etc.
Keep in mind when I post about rifles, I always look at it from a hard use/cobat ready/work perspective. Buy whatever if you never plan to train hard, use it for work or use it for defense. Hope this helps some.