There are two ways to do it actually. The first is buying a conversion kit. It's definaltely the cheaper option. The second is to buy actuall Hella 90mm hid units for both high and low beams and replace the halogen ones with them. They sell them at rally lights and it runs almost $500 per light. That'd be almost $2k for a full setup, which is spendy; but it would yeild a superior beam pattern.
No wait, to put HID in the low pros it should be very easy!
You just have to buy the right kit for the H9 and it should be straight replacement!
The HID kits for the H9 have the light in the same spot (not deeper) so as a result the beam is the same, just more bright!
As was stated earlier in this thread, there are several ways to add Xenon/HID's. The first, cheapest would be to buy Cloated Halogen Bulbs like Sylvania's. They should improve lighting and give somewhat of a Xenon color. Now for the real options, the most cost effective would be to purchase an H9 conversion kit. This method would be pretty simple. Replacing each Ballast in the Moss kit would most probably produce some nice light, but how practical is it to spend so much money. Check out this site, it is really informative for lighting conversions of all types...http://www.hidplanet.com/forums/ As far as converting just the Low-Beams to Xenon/HID, that is realistically the best solution. Even on high-end vehicles equipped with Xenon/HID, the High-Beams are usually Halogen. Because the High-Beam is used to flash, it needs to be instant on and off. Xenon/HID bulbs take a few seconds to heat and fully light up, not practical. On BMW's, take a look at their Xenon/HID Angel Eye design. The outer ballasts are the HID low beams, the inner ballasts are halogen high beams.
I would love to see a NA with a Moss Low kit and HID's
With a bit more reserch, I learned some interesting information. After reading this, you may reconsider how you go about fitting HID's to your Low-Pros. I may just invest $1000 for the lows, and leave the highs as halogen. READ THIS...http://www.hidplanet.com/forums/viewtopic.php?t=3505
After quite an extensive bit of research on HIDPlanet.com, I have learned that there is really only one way to properly add HID's to the Moss Headlamp Kit. While the Moss Kit comes standard with Halogen Projectors, they are insufficient to properly run HID's. The light, because the projector lens is different, will have hotspotting, the cutoff will not be allighed properly; ultimatly, one will have the HID look but the lighting quality will subpar. In fact, using an HID kit designed to replace the H9 Halogen bulb would probably produce results worse than just using a standard H9 bulb. H9's are really good halogen bulbs.
What needs to be done, though it will be expensive, is the Halogen Projector needs to be exchanged with a HID Projecter. I know, they run about $500 each. Well, that is what needs to be done. After exchanging the module, a HID kit needs to be purchased, though not the ones on eBay. The eBay kits are poorly made, don't last long, and are just direct replacements to halogen bulbs. On HIDPlanet.com, they sell kits designed for those that have an HID projector. Considering you have added the Hella HID Projector module, you can use a real HID bulb, a D2S. All the cutoffs will be alligned, there will be no hotspotting, and it wil shine nice and bright.
It all depends on what you are trying to do. If you are just going for the look of HID, get a cheap eBay kit with a H9 bulb. Otherwise, incour the expense and do it right. When done, you will have a beautiful setup that rivals the likes of a BMW or Audi HID setup.
I will probably try one of the cheap ebay kits on my hella h9 projectors. A few of my buddies got the $150 ebay kits, and they look great, the light output isn't bad either. For $150, i'll buy a set and see how it works with the hella h9 projectors. I will be going with the 6000k ones.