In addition to Neil, get in touch with Farrukh Majid-he's done as much or more fiddling with the spring/shock setups on later W124s (Neil's car is an earlier W124, which had different springs & struts to begin with) than even Neil. Currently, I think Farrukh settled on 500E front springs & struts, as well as 500E springs at the rear. Remember, a 500E has self-leveling too, and weighs just a bit more than the wagon. If you go all the way into an MBZ Sportline or Bilstein Sport setup, ride quality will be severely compromised.
That said, if I were you I'd first look to replace my rear air cells, then see how it rides. If still too soft, I'd do 500E springs & shocks in the front as a next step.
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car