DAS220D,
I think putting a Datsun transmission in a older MB is a very reasonable idea. A 220D could benefit from another gear, as the engine is working at like 3500 rpm in top gear at 60 mph. Maybe a Datsun tranny could also boost the 87 mph top speed, but that maybe wishing for too much.
If you had a 190E or a W124 there would be a lot more enthusiasm for mods, but on older MB people just don't bother with that. Part of the reason for this is that MB never made performance parts, etc to promote people to tinker with their cars until later on. Datsun started early by offering performance parts for cars like the original 510 and taking part in racing that made mods quite popular with owners.
A 220D will not ever be worth lots of money because it's a diesel and too many where made. It's the same thing for most MB sedans, for example MB has already made more then a million W210's so unless you have a E55 you can't expect collector car values for it when it ages.