wondering what the lowest I can lower the car without causing damage from, perhaps, a dip in the freeway, speed bump, driveway, etc...

secondly, how much would it cost to lower the front and back?

I would be happy with just getting the top of tires level with top of fenders at stock stance...

airbags all around eventually, but thats a wayyyyys away

 

Using Lowering springs is about the safest you can go without problems. Pedders is the best[some will disagree], and another thing is, if you go too low, by using other means, the axle shafts will be at a angle that will cause problems.
My personal prefferance would be Pedders springs, and be happy with that. "Moon"

 

I bought a set of Gold Lines front and rears used in the classifieds here.

They come up for sale used now and then, keep a watchful eye for when a set is offered.

A few Before & After pics.









Buying a gently used set (low miles) makes it a pretty cheap and Easy to install lowering Mod.