I agree with engatwork with the additional consideration of the seat pads (between the seat springs and the seat cover material. I have done at least a half-dozen of these over the years.
If the springs are good (you can reach underneath and carefully check each spring wire), then at least the pad needs to be replaced. engatwork right though, new springs with new pads are simply great. Incredible actually.
The pads have been greatly improved over the years. They are form fitting and are rarely exchangeable from one car model series, but there are some exceptions.
Check the edge bolsters (on each side) if they are not pronounced, designed that way to hold you in the seat, the pad is shot. Additionally, if pushing down on the front edge of the seat does not offer very much resistance, consider springs and pads. If you seat belt latch area starts squeaking from the seat edges rubbing against the plastic, you are often in need of new pads.
For older cars, often the seat back pads can still be used, especially since the new seat pad will elevate one back up higher on the seat. Compare the front passenger seat to the driver's, a good way to see how bad the wear has become.
There have been several good posts on the topic of repairing seats. Use the search feature. Here are a few: Seat Repairs
This is a repair that although takes some work is one that I really enjoy the results of. Mercedes seats are truly remarkable parts of the cars. For a car that you truly like and look forward to many future miles, do at least the pad and consider the spring. Your other end will thank you.