I've been looking for a near new seat for years. I've given up and ordered the spring box, seat frame and pads new from MB. You can buy a Heavy Duty option (for the spring box and pad) to help stop the dreaded seat sag in the 124's.
The leather covers are no longer available from MB as far as I know, but I think other members here have received decent ones from D-class in the past??
Is the car a keeper, or is it just getting a birthday - ready for sale...?