Don't let the engine computer keep you from a slightly later model MB Diesel. Through 95, except maybe for the 95 E300, disconnecting the computer will not leave you stranded. At worst you might have to bump up base idle to keep the engine idling, and on most 90s models you have to convert the wastegate from vacuum control to pressure control as nature intended. That reduces demand on the engine computer to controlling idle speed, which rarely goes bad, and emissions functions. Climate control is a bigger puzzle on these later models.

Newer models are relevant if you're after mpg. Based purely on what I've read, you have to work harder to get 35 mpg out of a 240D MT (no, MT isn't part of the trunk lid badge ) than a 90s 300D 2.5 AT.

A 190D might not be a bad choice if you're Napoleonic in stature. Those are commonly available with MT.

MBUSA didn't offer 240TDs but I've seen a couple on the road.

Given your specs and level of enthusiasm, I say find a 78-85 300TD, turbo or not, and do a MT swap. Riding your wife's bicycle will either put you in a better habit or accelerate the swap

Sixto
87 300D