1.) I wouldn't waste the money on a Pre-Ban Lowers because: (a.) the law will sunset soon.
(b.) whether or not it sunsets..if the SHTF happens just perform the necessary modification.
Instead you should buy some extra magazines, because of additional laws that have been passed which restrict these items.
You should also seriously consider buying some spare parts and reloading equipment
And you should be thinking about buying a 50 caliber rifle (if you can afford it and the ammo).
At the minimum your personal "armory" should have the following:
1.) Semi-Automatic Rifle with 15+ magazines and spare parts: the excess magazines are nice if you are forced into carrying an excess load in ammo or if existing magazines where out.
2.) A precision scoped bolt action Rifle. Capable of hitting a man sized target at 1000 yards.
3.) A good shotgun
4.) A good handgun in an effective defensive
caliber.
5.) basic gunsmithing tools (screw drivers, files, etc.)
6.) spare parts, solvents, etc.
7.) Basic reloading equipment.
8.) Load Bearing Vest, suitable camoflaged clothing for your environment (if it is a city, shirt and jeans! :-) ).
9.) a cheap but good range finder.
10.) Binoculars, Night Vision Gear if you can afford it,