My 1982 300D has a set of KYB shocks all around put in there by one of the previous owners.
They are fine for me - they hold the car up fine - the ride suits me OK. I'm not too much into performance with a 300D.
Bilsteins are 15% off at Performance Products on a special sale they are having.
Not having any experience with Bilsteins in a W123, maybe I don't know what I'm missing.......