There was an article in Star magazine about replacing the springs and pads in w124 seats. The author concluded that the mechanical labor was not difficult, and identified the various connectors, fittings, etc. If I remember correctly, he concluded, that reinstalling the upholstery so it looked decent was a tough proposition, and he wound up taking it to an upholstery shop to have them redo his leather re-attachment, which apparently had too many creases and puckers after he did it himself.
I looked into having it done, and was quoted $320 for new springs and pads. The shop had done a number of them, and said that about half the customers didn't like the firm feel of the new seats, and said they wish they had just put a pad under some sheepskin covers. I am still debating, but intuitively would rather put the money towards something else.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E