This may be the best route. I'm sure with some modern memory foam and some new springs and bolsters the seats could be made to be quite comfy. That being said, electric w126 seats are pretty reliable and may be a cheaper\easier option.

One thing I love about the electric adjust seats in the 190E is the tilt feature. I get better thigh support this way, where the W123 manual seats do not have this adjustment. The springs are still pretty firm and the cushions have been repadded.