Well I love the 1984-85 500SEL's. That engine has the dual-row chain, but the main thing to look for is if the timing chain guides/rails/tensioner have been changed and up to date. If not or if last change is unknown, highly consider doing that.

Other than that, they are basically the same chassis as the 300SD, only on a stretched wheelbase. I had a '85 380SE (gas engine), which was a smaller engine, 275K miles and still had good power. One of the best MB designs IMO, but I would say that since the W126 are my favorite Mercedes chassis.

Good luck!