I would simply use the factory Sportline parts for the springs, sway bars, front control arm bushings, and rear subframe bushings. After you have that done, if you want it even stiffer, then consider upping to the 500E and EVO sway bar setup, or race springs, or something. But just rebuilding the suspension completely to Sportline specs - AND putting on some fat wheels & tires (16 or 17 inch, 7.0-7.5" wide) will make a HUGE difference while not totally killing the ride quality. (Hey, if you want a go-kart that seats 4, buy an old M3...
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A copy of the Mercedes EPC (dealer parts catalog on CD-ROM) can be a big help since you can get all the Sportline part numbers. The guys at FastLane (link above) can probably help too.
