Before totally condemning the springs -- check the trailing arm bushings very carefully (can be hard to see). I'd think that with accumulated mileage and then the introduction of heavy loads the rubber bushings would give up before the springs in just a months time. Bad TABS will cause the rear to sag.
You probably do need overload springs though and my experience with a set (unknown source) led me to change out a good set of standard rear shocks for Bilstein HDs on my '80 245. That rode too stiffly for me unloaded and skittish in the wet - left in the HDs and went back to standard springs as a happy compromise for me. - Dave

Back in the day, I redid the suspension on my daughter's 760 Turbo wagon. The Nivomats on the rear had given up the ghost. She was in a band and hauling guitars/amps was the order of the day. Put in Bilstein touring shocks/strut inserts and IPD's overload springs in the rear. Since the Nivomat contributed to spring rate, the stock springs used with those would have been way too soft.

That combo worked beautifully - especially when loaded. A bit firmer when unloaded. However, unless you go to some air-adjustable device, you can't have your cake and eat it too. Firmer springs to help deal with heavy loads are going to ride firmer when unloaded. If you go that way, suggest removing the rear sway bar if the car has one, and/or going to a larger front sway bar. Of course, tired bushings front/rear should be replaced.

By the way, the Koni Sports have a LOT more damping than the Bilstein HD's....

TegelMike,

This is my opinion, and my opinion only.

I've done this on three of my four 240's.

Rear:

NO OVERLOAD SPRINGS FROM IPD, EVER! Ask me how I know!

Biltstein HD shocks for the rear with Volvo OEM springs. The ride is amazing as is the handling.

Front:

Biltstein Touring OR HD Struts for the front, though the HD will not fit in your strut tube, so you are stuck with the touring, if you chose to go that route.

As far as bushings go, I prefer poly over rubber, but that's your call.

Matt