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.
Dreamers, misfits, malcontents, finding the world not to their liking imagine themselves ideally suited for the world as it ought to be. - anon