Stocking up on lowers basically works like this:
You stock up on lowers. (I'm getting 2 more for a total of 3 lowers)
If a ban happens, it'll usually have a point where the serial numbers will define the start of the ban. Anything previous to that SHOULD be grandfathered in. Since the old serial number lower is grandfathered in, you can have anything that you want on your weapon as if it were today (4/11/08).
Since a lower is only $100-$200 each, for the cost of 2 fully functional ARs (Say $900 each x 2 =$1,800.00) you can get 12 AR lower (est $150.00 each) Once you have the lowers, you can build them at your leasure, when you have the time and the $$.
If a ban does not happen, you should at least break even on the cost of the lowers.
Problem is, there will be people looking to buy up lowers so that they can gouge others in the cost of the lowers if a ban does happen. I suggest that you buy what you need to build rifles that YOU want and add a couple more lowers for builds for your kids. If that means you buy 2 for you and 2 for your kids, that's fine. If that means 5 for you and 30 for your kids, that's fine too.
Hardwarz