Crusader,
To answer some of your questions:
1) The receiver is THE piece for determination of whether or not the weapon was "pre-ban." If you want to build something wild and exotic (meaning that you'll be putting some "evil" features such as a collapsible stock, flash suppressor, etc), you'll have to have a receiver that was part of a complete rifle that was built and sold prior to the ban going into effect.
2) I'm in the process of building an M4 look-a-like. I started with a pre ban lower (they cost anywhere from $750 to slightly over $1000), and have added a M4A3 flattop upper, 6-position stock, and a Phantom Flash Suppressor to the 14.5" barrel. All products were ordered from Bushmaster. Expect a 3-4 week turnaround on uppers. Their website is helpful.
3) Again, expect to pay for a preban lower. A postban lower with a preban upper is worth $10,000 (fine), and 10 years in jail.
My $.02