Pick a lower you like and buy a lower parts kit WITHOUT the fire control group. Then find a trigger and stock you like and you should be gtg. I'm a billet lower snob and am partial to JoeBob's as a source. He has good deals on lowers (some allow roll pin free assembly). He also has good prices on lower parts kits. As to triggers, I run Giessels on most of mine, although I do have an ALG on one. I prefer A2 stocks and RRA make one I like a lot, the Operator A2 model.
As to assembling all of this, google/YouTube can be your friend.
A warning is in order here. Notice I referred to "most" of mine? These things are like Lays potato chips, once you start fooling around with them you may have to join BRA (Black Rifles Anonymous) to kick the habit.