No idea who told you off, there are nicer ways to suggest moving to the DD subforum. This is definitely the right subforum for talking about diesels though, there's quite a brain trust in here and the guys who know everything rarely venture to the other subforums.

1: You'll need to replace the front main oil seal. I'm not well versed on the 617's in that area, so you'll need to wait for one of the 617 guys to fill you in on the details.

2: Change the pump. It isn't worth messing with just the bearing. Buy a Graf or Laso pump with a cast impeller. It'll last you another 30 years.

3: I assume this is a manual? If so, you need to rebuild all the shift bushings. MB gear shifters never had a great feeling from the factory and a worn out one is simply HORRIBLE to drive. There are aftermarket kits to replace the rubber bushings with Delrin plastic and is said to greatly improve the shifting experience. If you search this forum, there is a writeup on how to rebuild the shifter, including pictures (assuming they weren't hosted externally and now gone).

4: The heavy feeling is normal for a W126. It shouldn't be set in concrete, but it should still require a bit of effort. If coming from a modern car, it would be easy to think something were wrong. Just keep an eye on the fluid. If it's murky and brown, change multiple times until it runs clear. ATF was the original fluid and is what should go back in it. There's a filter in the bottom of the reservoir that probably needs changing too.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]