As far as repairing the hydropneumatic shocks, I've seen it pointed out elsewhere that they are just hydraulic cylinders with mounts on them to attach to a car.
Since they are just that, you can often take them to a hydraulic repair shop and have them rebuilt, where you couldn't do the same for a "normal" shock absorber.

All that being said, I've never worked on hydraulic suspension, as there is none on either of my cars, even though the 500 claims to have it front and rear if you believe the VIN!

Best of luck, and happy wrenching!

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...