I don't see a problem with option B. I was even going to suggest that if you don't have more than 400-500 at the moment you could buy the weapon in stages if you are willing to pay the 600-900 necessarry for a quality rifle. This option might be a real blessing to you if the legislation takes a turn in our favor next year. In short, you could shell out about $250-400 for the QUALITY lower, and it's corresponding parts. Save another $300-500 for the upper and the stock later on down the road. If the legislation takes a turn for the better, then the stock you end up adding might be a 4 position collapsable and that upper a 16" with a threaded barrel, bayonet lug and flash suppressor. Now you have the weapon many have waited for a loooong time, and no money was wasted on a post ban barrel that you will end up replacing, etc.
If you are firm on the 400-600 price range your best bet IMO would be to buy a quality kit. Talk to the guys in the DIY section for their opinions on what might fit your budget.