I have a Mercedes 300SD, 5 cyl diesel, auto. All they came with was auto. Some 5 cylinder-o-philes have adapted the 4 sp manual from the 240D of the era, the bell housing is similar, IINM. If I was rich and eccentric, I might spend major bank to have one built but holy crap, that would be hard to pull off in a home shop. Just getting the clutch pedal right would be a challenge. It's an awkward place to work, even with the seat out. Just the work to attach a framework to support the hinge of the clutch pedal would be a major chore. Maybe I'm getting too old, not sure.
As I know more of mankind I expect less of them, and am ready now to call a man a good man upon easier terms than I was formerly. - Samuel Johnson