Paul,
Don't even think about air ride ... just get coilovers they go from about 1.5" drop to 3" drop (have to roll the fenders in you have 265/19 or wider in the back).
FK - harsh ride
H&R - fairly new, haven't read/heard any reviews ... should be a good setup
KW Variant II or III -
Here are some other options if you don't want it too low for the winter
http://www.bmwdiy.info/suspension-survey/
I may be in on the group buy ... depending to the price