The self-adjusting shocks are the same price on a SL 500.
1993 is not the only year for the 600SL. The badge was changed to SL600, and production continued through 1995. I can't imagine why parts would be different from '93 to '94 and '95. Just the badge changed. Here is a link to the MB USA website that shows production figures for each year:
I think all of your comments are valid about having the car meticulously checked out. The maintenance costs of these SL's will be higher than other cars. But there are no expenses in a SL600 that wouldn't be incurred in a similarly optioned SL500. They also use the same fuel injection system, LF-SFI (hot wire mass air flow sensor, sequential fuel injection).
I think the running rich thing ruining the engine is from the isolated example of the guy in Montana who has an S600.
O2 sensors, fluids, filters, etc. would be checked / changed in any car regardless of the engine.