It feels like the w126 seat is higher than the w123 seat.
If they can't do that, ill most likely try just plain grey/black seats on blue or maybe light tan on blue. I have found that the best way of doing this is to completely recline the seat and then stuff the upholstery backwards through the crevice between the seat bottom and back while tilting the seatback back up. I think that would look great. The w126 seats are massive, I'd guess 80-90 lbs each. Even when lowered all the way, it won't go as low as the original w123 seat. I went to the W123 forum and did a 'search' on seat height adjustment and found several discussions. Interior Auto Parts for Mercedes-Benz 230/240/280/300 (1977-1985) W123: Carpets & Floor Mats, Dashboard & Steering Column, Interior Accessories, Interior Trim, Panels & Doors, Seats, Belts & Upholstery ... You will have to remove the electric seat tracks for the W123 manual one`s. Later in 1976 coupe and long-wheelbase sedan models were produced, and in 1978 the first of the famous W123 wagon models was sold.
Part of the popularity of the W123 was that it was released in such a wide variety of models and options--different engines, paint colors and upholstery.
The w123 seats are pretty light for such a large seat, I'd guess 30 lbs each. Here is …
Out of OEM or upholstery style fabrics, we manufacture some very popular Mercedes 240 D (W123) seat covers like the Scottsdale seat cover, the Madrid seat cover, and the Regal cover. Look in the W123 forum for details, questions and answers. The interior of a car is more than seats and doors - although that is a lot of it, and we at Adsit Company sell a lot of Mercedes Benz seats, seat covers, door panels and new doors.