Hi Brian,
it has been very interesting reading all the comments. You are correct in what you say about the ratios, so I am absolutely waiting to see how you go. I have a 1991 300SE and it always starts in 1st gear. My previous W126 was the 420SEL and I got an electronic control unit from a guy in Ireland for around $200 and it worked very well. It used the ABS pulse for speed to change gear. You could adjust when you wanted it to change.
I will be waiting to see how you go Brian, best of luck
BrianD.
Ps.
420SEL does not have that much power, my old Jag used to run rings around it. (;-)