Hi, i posted a thread a few weeks ago asking if there was a cheap way to convert to Xenon. Didn't get much of a reply. Anyway, a few weeks on and i have converted my W211 from standard Halogen to Xenons and they work fantastic. Pics will follow naturally but the sun is shining and i'll wait til later.
Basically, they cost 209quid plus delivery from a UK company and OEM suppliers of the ballast units. Basically you get the two ballast units for LHS and RHS lights plus the Xenon bulbs. You specify fitment, in my case the H7 and the Xenon bulb comes with that fitment. The hardest thing on my car was trying to find some damn space to fix the ballast units. Anyway, whole job took an hour and it is very very easy. Bulb failure due to difference in resistance did not cause any issues.
Anyway, here is a link to the company i used, they have many options but you cant beat the real mcoy and a proper HID conversion is the way, the difference is outstanding which i realised after having changed just one bulb and compared the difference.
http://www.hids4u.co.uk/
Regards, Harry