Coilover Pros/Cons

[we350z]

Debating replacing my stock suspension on my 2000 4.4i (rear air ride) with a coilover type due to ongoing issues with it. Couple questions:

1) What is the typical height range adjustability?
2) Will lowering adversely affect towing capabilities
3) Will lowering adversely affect snow driving
4) What all else should I replace and refresh while i'm "in there"? What bushings are recommended? OE? Energy/Prothane poly? Powerflex?

I have a Thule roof rack that is low-profile but still does not clearance some garages. For this reason I would like to drop the ride height a little. Also should improve handling a bit in addition to what I assume will be a firmer ride (especially with Swift springs).

I'm assuming just the airbags need to be removed and the new coilover assembles mounted in their place in the rear. Do I need any other hardware if I go with something like the BC Racing kit?

I've heard you can then either live with the "Self Level Susp. Inact" dash cluster warning or code it out with diag software.

I found a local BMW independent shop that would do the install for $400 + $100 alignment. Seems reasonable to me. I do have a garage but this vehicle is too big to work on in it.

THANKS!