Do a "trust" and efile on your ATF form to get your tax stamps for that. Also, you'll need to find an FFL dealer that works with NFA items. It avoids doing it the old way of having to go to your local sheriff an get pictures, fingerprints, etc. You can put a pistol upper on a lower with a stock only if it's a registered SBR, other than that, it must be registered as a pistol. Buy your NFA stuff (supressors, sbr's, etc) while you can, it's not going to last much longer.
Unless you have $10-15k, I would avoid buying an M16.