All 500s and 560 SELs had self leveling suspension, the shocks rarely go bad, the accumulators loose their pressure. I you are handy you can change those for approximately $100 each, be prepared to get an oil bath. I have almost every tool, and still did not attempt it. A sign of bad accumulators is that the rear of the car bounces while driving. The shocks themselves are $450 each.
The fronts are normal Bilstiens, the only issue could be if the car has the adaptive suspension, which was an a option. Then the shocks on all four corners are controlled by the computer.
BTW who is telling you that you need new shocks, are they leaking, my dealer (MB) only replaced the acumulators, the ride is fine now.