Most likely obama will try some sort of feelgood stuf like this so as I said buy a lower before it happens and you wont have to follow all the feelgood stuff.
That's not 100% correct. With the Clinton bill, you needed to not only OWN the gun before the ban, but it already had to be equipped with the "bad stuff" before the ban. If you purchased a lower receiver only, and it didn't have three or more attachments on it (pistol grip, telescoping stock, hand rail system, flashlight), it was still considered a "post ban" gun. I went through that in the early 2000's before the ban was lifted. "Pre-ban" lower receivers were selling in the thousands, while "post-ban" lower receivers were selling in the hundreds. Some people who bought lower receivers before the ban, but were never equipped with the right stuff thought their $100 item just increased in price by a factor of 10. They were wrong.
So if you are concerned about a similar bill coming out, make sure you not only buy the lower receiver, but you deck it to the hills BEFORE the ban takes affect. But, if you are more concerned about being able to access a gun, buy up as many stripped lowers as you can. A stripped lower receiver on an AR-15 will set you back $100-150 if you get it at the right place (DPMS, Rock River). A complete lower receiver will set you back $250.
As far as selection goes, I'd get a gun that runs off nato rounds, or what the US military uses. If bleep hits the fan, there should be plenty of that ammo lying around. 9mm, .45, .223 will all be easily accessible. I immagine it wouldn't be TOO hard to find .308 or 7.62, but you'll probably have a hard time finding .300, 30-06, .357, .40 or the like.