I don't disagree about the rails - they were dark and brittle. There are differences of opinion on the sprockets, however. As mine showed no wear (much of the elongation was in the tensioner and rails) my mechanic did not recommend replacement. He typically doesn't want to change things that don't need it. He is a well known and respected MB mechanic in these parts so I usually follow his advice. And a machinist was with me when I went to look at my disassembled engine, and he agreed.
I respect your opinion and you may get different results in the end.
Good luck with you sale.

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John Gillespie
1988 560sec - 192k miles
2006 CLK500 - 40K miles
1995 E300 - 202k miles