What is the lowest I can lower my HHR without damaging something when I go over a bump or something on the freeway?

Top of tire with bottom of fender? (basically level)

I see these 1" and 2" kits for $200-$300 and I think to myself, what is the point? Seems like a waste of $$$$$ to only drop it that small amount...

Also considering getting bags. But I can't get a whole kit at once, so it would be tank, bags, etc, etc in increments. Obviously getting the parts I can use first (bags, tank, controllers) and then getting the compressors and auto leveler and what not later down the road.