As stated, the receiver has to be virgin and form 1 to the BATF as a pistol receiver. So just buying a standard receiver is not going to cut it. The FFL that builds/machines the receiver will have had to register the receiver as a pistol receiver, not a rifle receiver.
So once you get the pistol receiver, you will need to weigh the receiver and the parts. The built pistol cannot be over 50oz, due to the mag not being inserted into the pistol grip, but a location other than the pistol grip.
So step two is to mill the receiver to loose the extra weight and get you to/ below the 50 oz max weight allowed. It can be down, but leaves a lot to be desired. Also, since the gun is a pistol, no butt stock, or any forward grips. By adding these items, you make the rifle a SBR (butt stock) or AOW (forward grip).
[img]http://www.quarterbore.com/images/oa98_a_03_small.jpg[/img]
If the pistol is built on a Post ban pistol receiver, then adding a FS is out.
As far as owning a AR-15 pistol, no special tax is requried, it just another pistol in the eyes of the BATF.