is there a rebuild kit for the rear bench on a w123? or is it more cost effective buying a used reconditioned one? thanks.

I want to know how to rebuild the spring supports on the front seat of my '85 300D, so if you find a way please tell me. Thanks.

The pads on MB's break down and need replacing. Do this before the covers are damaged, they are expensive. The springs on the drivers seat bottom will break down in time, but the rear springs should be fine.
If you are a back yard mech, replacing the pads is not a bad job.
The pads can be purchased at the dealer. I would do both rear pads as long as you are working there. Most upholstery shops are familiar with this job and the cost is reasonable. Ed