Before my current 540 BMW, I drove (and still have) a 84 928 (bought it in 87). It has the same trans as in a 500SEL (more or less) except it starts in first gear. Its shifter was better for manual shifting than MBs and I used to shift manually quite a lot both up and down. Its harder on it than letting my mother drive it but its the way it was designed.
And get the shift kit. I book marked this link as I lost the last one that appeared here. I have done my own many times but the ability to use engine rpms for some of the control (as the other unit did; and I suspect this one does) eliminates the hardest part of my system; which used a pressure switch in the governor test port to gain such control.
Bosch Master, ASE Master, L1
33 years MB technician