Here are a couple existing threads for you...
http://www.ar-15.co/threads/75119-NF...El-Paso-County
http://www.ar-15.co/threads/60985-Pe...with-NFA-items (let us call this a "lively" discussion)
Reader's digest version:
1) Make sure the lower works before you invest in a $200 stamp for it.
2) If you're interested in looks, engrave it first so if the engraver screws up, you can get another lower.
3) You can get the barrel, just store it with a friend who doesn't own any ARs while waiting for a stamp.
4) Then do the stamp.
O2
Ps. You're right, once you put a butt stock on it, you can't go back to a pistol.