The toddler seat (20-40lbs) is not available from Mercedes-Benz. We used the Mercedes infant seat, and it's a fantastic unit, but were disappointed that they did not offer the toddler seat.
However, I found the Britax seat, and bought it. It's an outstanding seat. We bought the Roundabout and used it purely as a front facing toddler seat. Yoiu might want to use the tether anchor system, though not required by law with your 1999MY car, I think, in the US. Your dealer should be able to install the anchor.
Now, my four year old uses the Mercedes booster seat designed for 40lbs to 100lbs. It has the BabySmart system, which is not often used as I don't let her ride in the front, but it's nice to know it's there in case I have to.
The Britax seats have racing-style impact foam instaed of plain plastic with a cover. My daughter made many long car trips in hers and we found the high comfort level worth every extra penny the Britax costs.
1998 C230 "Black Betty"