If you can find a 240D 4 speed for a parts donor car, swapping to a manual is fairly straightforward. You'll need the transmission
along with the shift linkage, hydraulic setup, clutch assembly including flywheel, and you might need the driveshaft as well (don't remember for certain).
The slushboxes are decent transmissions on these cars; just, if you decide to rebuild the one in there, make sure you find a shop that knows M-B's, and don't skimp. A friend of mine bought a "good used" automatic for an '81 300SD, and ended up regretting it...the transmission leaked like a sieve and, other than the torque converter being in better shape, it wasn't really any better than the one that came out.
Depending on what shape the front suspension's in, that might not be a bad deal..however, keep in mind the adage "There's nothing more expensive than a cheap Mercedes"
At the same time, I ran cheap M-B's for 9 years and had very good luck with them