Fitted a HID kit to my 90 series last week and it is a lot better on the road.
I did a lot of research so I'll give you a few pointers...
-55watt systems produce more light than the 35watt systems HOWEVER the bulb life reduces with that (but still last longer than a normal halogen)
-3500k bulb - Tint of yellow, bright! 4500k Bulb - White, Brightest option, 5000k - hint of blue, bright! -6000k, Blue, still bright - bassically the higher you go the more colour you get, the less actual light you get.
-Some of the kits on ebay are good, some are rubbish.
I'll have some pics up soon!