300SE uses the same M103 engine 722.3 transmission combination as the W124 300e.
W124 260e uses a M103 engine with a 722.4 transmission.

As mentioned previously. The transmissions let these cars down in the comparison of modern vehicles. The transmission is a 4 speed albeit it's set up to start in 2nd gear.
4th gear is 1:1. To select first gear you either need to select it with the gear selector when stationary or give the accelerator the full boot at low speeds.

A 420 does not use much more fuel compared to the 3.0l engine - it doesn't need to work as hard to get the car moving.

Everything pertaining to the M103 engine and transmissions in the W124 300e and 260e are common with the 300SE.