I can tell you that free float means just that; it doesn't make any contact with the hand guard or stock, it "floats." Rifles with free float barrels tend to be more accurate.
As far as kit vs. complete rifle your paying for labour and usually a warranty on that work. Don't forget that you pay tax on both the kit and the receiver plus any FFL fees and paper work for said receiver, and possibly shipping for one or both.
I'm not an AR guy but BUIS has something to do with the sights. "Back-up Iron Sights" I think.
As far as kit vs. complete rifle your paying for labour and usually a warranty on that work. Don't forget that you pay tax on both the kit and the receiver plus any FFL fees and paper work for said receiver, and possibly shipping for one or both.
I'm not an AR guy but BUIS has something to do with the sights. "Back-up Iron Sights" I think.