Traxxer said:
H&R spirngs - about $320 shipped. They claim to lower 1" all around but from my experience the drop was more lik 0.8". Ride comfort wasn't terrible. Handling was slightly improved, and less body-roll compared to stock. Wheel gaps were still too big IMO so I swapped them out.
Eibach springs - $300 for the springs and another $300 for the shipping from Germany. You can order them through Eibach USA. Drop is said to be 0.6".
H&R coilovers - about $1500. Only height-adjustable. You can't change rebound/compression settings. I don't have experience with the Z4M application, but if the kit is anything like the one for the regular Z4, E46 M3, E36 M3, then they'll ride pretty rough and not comfortable.
KW V3's - about $2600 shipped. Very nice coilover kit. You can change rebound/compression settings individually. Very high build material and quality. At the softest settings, the ride comfort is pretty close to stock due to the progressive rated springs.
TCK coilovers - $3K shipped, including camber plates, rear-shock-mounts, and RTAB's. I have this kit, and I've also told a few people that I can no longer recommend TCK to anyone because of the trouble I've gone through with their products and because of their lack of quality-control. My kit is now finally working properly, only after a lot of wasted time, engergy and dead brain cells. On the other hand, once the kit starts to work properly, I have to say the handling performance is simply awesome. The TCK coilovers have a better ride comfort than the stock suspensions.
