The springs of the early W124 tended to break down after lots of km's/miles, but the later models ("89,-->) did not suffer that problem anymore.
The cushions/padding can be exchanged for new ones quiet easy. The new padding isn't that expensive and the work necessary to get it exchanged can be done within 3 hours (if you know what you're doing) or within a spare saturday (when you take it as an experience).
The seat has to be taken out of the car and the "back-rest" has to be removed from the seat itself. The cables and motors in the seat may make it look difficult, but...
It is not that hard to exchange it.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)