I will expand on some of your SDL points:
For the vacuum pump inspect it first, if it has 4 screws on the cover you are fine. If the cover is smooth that is the old type with exposed bearings. If it lets go it will cause the timing chain to snap, and you are looking at a rebuild.

If the engine is noisy it is probably the lifters, run a good quality synthetic oil and see if it helps. If it doesn't replace the lifters.

Doing a diesel purge once a year is overkill, and IMHO I would not do one on a diesel with a fresh set of injectors. You can cause more harm then good.

W126 front end clunk:
Could be a few things, but the tie rod ends and guide rod mounts are very commen.

W126 rear end clunk:
Check the sway bar links, they make lots of noise!

__________________
2006 CL500
2009 C300 4matic
1969 280SE
2023 Ram 1500
2007 Tiara 3200