Re: AR 15 BUILD
I have built several AR-15s.
I like Spikes Tactical lowers. I have found them to have better finishing than others at the same price point.
I like DPMS for Lower Parts Kits. They have the most consistent parts and nicest triggers of the kits I've used. I've found that the 'cheapo' kits always have a part or two that pushes to either end of the 'spec' tolerances, making assembly a pain.
As for uppers, I suggest buying a complete upper. There is little/no cost savings building your own upper unless you already have the tools. You don't need special tools for assembling the lower (except a .25 inch dowel with a small hole drilled in it).
Check out Model 1 Sales for complete kits. I have a couple of very nice rifles built from their kits on Spike's Tactical lowers. My experiences with some of the other kit sellers have not been quite as good(del-ton, blackthorne).
Good luck with whichever you choose.