Unfortunately there are many people who can't really drive a stick with proper care. I knew someone who had trouble with every stick shift car they ever had. They still liked stick shifts, but never understood that they weren't handling it right.
Modern synchromesh transmissions will shift without COMPLETELY releasing the clutch. Because of this, some people develop the habit of slipping to the next gear without completely releasing the clutch. It's an easy habit to fall into and it will wreck the brass and sometimes worse. It's worth developing the habit of having the clutch on the FLOOR before moving the shift lever.
I expect that the PO developed the bad habit.
As anyone who reads my posts can tell you, I am a huge fan of MB manuals. To me it would be worth saving this great machine, even if it meant rebuilding the transmission. Someone said rebuild the whole thing while you're there. In almost all cases of synchro trans rebuilding, you only need brass, gaskets and bearings. I'm sure that's big bucks for one of these transmissions, and I hope I never have to find out. There were about 700 US manuals in '86, but only about 350 US in '88 and the '88 transmissions are different, so I'm sure parts for mine will be double hard to find and afford.