I know exactly what I want.
I've been sitting on a built lower receiver for the last 6 months and I have absolutely no clue as to when I will find the upper or exact upper components I need to complete this project.
Thankfully I already have several others that I can shoot while I wait.
If you want my opinion buy a full rifle to start.
Either on the EE or new from a dealer. There are some really nice rifles out there such as Daniel Defense or Armalite from Wolverine Supplies, LMT and PWS from Questar, and Stag Arms from Arms East to name just a few.