I'd be weary about purchasing ebay brand coilovers. I'd recommend that you check out the BMW E39 ST V1 coilovers by KW Suspension if you're looking to keep cost down yet want something that is quality for your BMW.
The ST V1 will give you the ability to adjust the height of your car with pre-set damping from the engineers at KW. This is perfect for an ever day driver and does still maintain a ride quality that is similar to the OEM Sport Suspension. Itís slightly stiffer and will definitely give you more responsiveness on turns as compared to the OEM setup!