Turbo 300d's are the most fuel efficient of the W123's.

Some 1981 W123's had a one-year-only trans that used vacuum controls but no bowden cable. I believe these transmissions start out in second gear as the older ones with the control rod do.(pre '80?)

I would think it better to stay with the transmission matched to whatever engine you decide to install.

If you install a turbo engine, remember to get the exhaust off the donor car , it is different.

With a turbo, you will probably want to go to the 3.07 or 2.88 rear diff also.

Dont forget to get the speedometer out of the donor car to match the rear diff. ratio