Noob looking for some parts advice

Hello all, I'm beginning to get parts together for my first AR build, which also will be my first AR. So far, I've purchased the aero m4e1 lower and aero slick side upper. Planning on a LaRue trigger and criterion barrel (cry once buy once etc), and probably a toolcraft bcg.

My hang up right now is the lpk. Since I'm dropping the cash on the trigger already, I obviously don't want to buy a worse one with the lpk. I don't need the trigger guard as it's built into the lower. I also would like to get an ambi selector and mag release, being left handed. Is there a good quality kit of everything BUT those parts out there? Springs, detents, pins, etc? Or am I stuck either buying them one at a time or blowing the cash on an lpk with parts I won't use?

Secondary question is the barrel nut.... Do these come with barrels or is that a separate purchase?

Thanks, forgive my noobishness, I appreciate the help!