i have them installed in my subaru, so i don't know much about how it will install in an x5

35w or 55w not really a difference

3000k is yellow
4300k is pure white
5000k is bluish
10000k is purple ---> rice

some ppl use the relay some do not... it's up to you, but i don't use the relays and when ppl have problems with their hids, they take out the relays and for some reason the probs go away, idk

like icer said... plug and play, very easy