This is really going to be dependent upon use. I have a couple lowers that are non NFA, one I use as a rifle configuration, and the other setup as a pistol lower. I then have several sbr lowers. I have different parts on different lowers to accomplish what I designated that gun to be for. For example one of my sbr lowers is setup with a LAW folder for off body carry. No need for the folder to be on the 16" gun. The pistol lower is setup so I can take my short uppers out of state with me, if I don't get the 5320.20 back in time.
I would suggest doing a separate lower, toss in a lpk and make sure things work properly, then file for that lower. After that its easy to get a factory assembled upper and be on your way. Also in the meantime of waiting for your stamp you can get an upper and setup the lower in pistol form to shoot it. Going with a factory sbr its going to sit in your dealer's safe for 8 months.