As noted elsewhere years ago, I replaced my Halogen Lenses with the OEM Xenon Lenses and it made a nice improvement in beam pattern with the aftermarket HID/Xenon conversions. Sulaco on Club202.com just asked me about the lenses I used, so here's my answer for the rest of the W202 crew:
The Xenon Lenses are not cheap, but they DO make a noticeable improvement over the Halogen lenses when using my VVME HID kits.
(I imagine that changing the housings would make a further improvement, but maybe only if the HID bulbs are the same shape.
The housings/reflectors appear to be only available as a complete assembly MSRP $1390, online $1084.20 each side!!!)
202 820 57 66 Headlight Lens for Xenon LH - MSRP $222, online $173.16
202 820 58 66 Headlight Lens for Xenon RH - MSRP $222, online $173.16
1999 MERCEDES-BENZ C280 Parts - Mercedes Benz of Fort Lauderdale
My upper seals were deteriorating, so I replaced them at the same time:
202 826 11 80 Headlight upper seal LH - MSRP $52, online $40.56
202 826 12 80 Headlight upper seal RH - MSRP $52, online $40.56
My lower lenses were VERY pitted, so I replaced them before putting HIDs there also:
170 826 09 90 Lower Driving/Foglight glass lens LH - AutohausAZ $18.89
170 826 10 90 Lower Driving/Foglight glass lens LH - AutohausAZ $18.89
The 4300K HID/Xenon Low Beams and lower Driving lights are VERY nice... very White and very Bright!
The lower Driving lights are naturally aimed low, so the beam pattern isn't a problem with the original lenses. I have all the 'parking' bulbs as LEDs now, and I always turn on the LED bulbs and the lower HID driving lights, even in the day time. I even get a credit on my vehicle insurance for 'daytime driving lights'.