Progun is right, AR15.com is loaded with info, lots of "i know better than everyone else" people over there, but most are pretty helpful, read all of the stickies, they will answer 90% of your questions...
another I will throw at ya' is ARpartsfinder.com .. i love that site...
Personally, I wouldn't trust a polymer lower on anything other than a 22lr, especially not on an AR10, but many, many others are building 15's with them, (I've just seen too many crack, and or massive failure in very short order).. but hey, to each his own..
(youtube videos of polymer receivers, and you will see tons of vids of catastrophic failures)
Billet v/s forged, pretty much no difference other than cosmetic. (uppers and lowers)
The military has been successfully running forged for as long as they have, so I have full, unyielding faith in them, and their cost is very reasonable..
Machined = very cool cosmetic lines, and look very trick, and with the anodizing, the finish looks cool, structurally, I haven't seen or heard of anyone that can legitimately explain why one is better than the other.
for pure cosmetics, mixing machined, and forged parts.. they generally don't "match" each other cosmetically, and machining processes or lack thereof.... but that's just looks.. function is the same.
the biggest thing I see from first time builders getting mixed up, is uppers and barrels not set up right with feed ramp /no feed ramp issues.. just make sure you buy corresponding parts, simple mistakes like that can be caught and corrected early on in the build... barrels come in 2 configs' for feed ramps, rifle, and M4, just make sure you get a matching receiver, Rifle, or M4, that will mate up accordingly, or you will have major feed issues, and it will drive you nuts figuring it out.. simple fix with a barrel swap, but its a real pain that can easily be avoided..
As you know, AR's like to be run a little on the wet side.. I have been using Nickel boron BCG's on my latest builds, and I have been very pleased with them, very slick, no need to over lube them, just a light wipe with oil, and they clean up in seconds when I'm done..
the phosphates are a little more involved in the cleaning. but function perfectly, and have functioned perfect for their entire history.
(I live in Arizona, and its very dry and there's a lot of dust in the air here, getting a wet gun all gritty happens very quick out here.)
Nickel Boron let me run mine a little on the dry side for me, that is a big plus.
I also prefer the "full auto" BCG's.. no real hard evidence on why its better, but with the added weight, It does cycle a little smoother than the enhanced, or the SP1.
there are studies that say it does help in the dwell time, and clocking, I have to agree with that, It feels smoother, hard to explain.
Personally, I stay away from the SP1's no matter what bargain price I see them at, I just am not to fond of having the firing pin exposed like it does, as it lends to a chance of the hammer dragging on it as it resets.
