Unless you buy a lower that was assembled into a complete pre-ban rifle, you cannot build a pre-ban rifle on it now. Do not pay the premium for a pre-ban lower that has never been assembled into a complete rifle as that is a sucker's sale. It does not matter when the lower was manufactured, if it was not assembled into a complete rifle before the ban went into affect, you cannot make a pre-ban on it now. The ONLY exception is if you can prove with receipts that you had all the parts for making a complete rifle before the ban became law, and only now are getting around to putting it together, and that is impossible to do since you are just now buying the lower.
To stay completely legal without any question, just build a post-ban rifle and work your butt off to get your legislatures to not support the renewal of the ban in 2004.
If you build a pre-ban now are you going to go to jail? No, there is no division of the ATFE that checks out every AR-15 seen in public to assure that it is completely legal, but if something happens that brings the rifle to the attention of the police, theft, shooting, any other violation that might draw attention, even mistaken identity, then you are in a world of hurt. Not worth the risk in my book. Just get to work on getting the bill killed, then you can do what you want. Do you "need" a FS and bayo lug, most do not, but I know I will be the first to put them on when the bill sunsets. I know I want my rifles to look the way they were intended to look, not like some neutered want-a-be. Besides, need has nothing to do with the errosion of our Second Amendment rights.