If the ride is stiff, chances are you need new spheres at ~$120 each rear, $180 each front for a 99-00 SL. A 98 S600 coupe only has rear SLS, I believe.

For an S600 coupe I'd try parts from a non SLS S320 or S420 sedan.

For a R129 I'd try parts from a non SLS SL320.

Sixto
93 300SD