The new spec'd 6.8 uppers haven't been out that long. Their new barrel specs should perform very well. Art Kalwas from SSA shot POF 6.8's at SHOT this year that was mentioned in another post. I think he had a favorable response. Maybe he'll be along to give you some details.
Any decent AR lower will work. Denny's has a new subdued billet lower that looks great. Spike's, AeroPrecision, Mega, SunDevil, RRA, CMMG...