My thoughts are: The springs are installed according to the weight of the engine. Thus a 240D is going to have lighter/softer springs than a 300D. The rears are very similiar.
I once replaced sagging rear 240D springs with ones from a 300D (non-turbo) and after deciding on the right spring collars, I was delighted with the results. Before it would sit low and ride low with light loads.
I would suggest first checking your spring collars. What bump rubber spring collars are installed front and rear?
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)