ADCO has RRA parts kits for $59...those are very nice, I've used a couple of those in recent builds.
As far as lowers, just get something forged; I've used Eagle, Ameetec, Mega, and another brand that I can't recall hehe. Anyway, I just get the cheapest one I can find...haven't had any issues with any of them.
The Eagle lowers for $80 each that Quantico Arms had a couple years ago was the bargain of the century. Those days are gone, though...problems with Armalite's QA apperently, so no Eagle lowers at all now.
The Mega lower is really cool with that nuclear logo it has...and I don't like the Stags because of their logo, but I'm sure the lower itself is just as good as anything else.
DPMS uses a really nice teflon finish, but they're more expensive than some of the others. Bushmasters are just overpriced, imho...you're paying an extra $85 for the logo...but I've never seen one, so I can't really compare the quality with anything else.