For what it's worth I also have a 98 E320 with 115K miles and my flex disks, front control arm bushings, and gaskets are original and doing fine. Other members have had problems with their bushings and flex disks and had to replace them. I did recently replace my engine mounts. My front and rear main seals are leaking but not bad enough to replace them at this point.

If you trust your mechanic then I would ask him the question "what if I don't replace the bushing and gaskets?" What was the outcome with the new flex disks? Does the car shift correctly?

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles