My best advice would be to tell your son to look for a 2000 model year or later. 1999 and earlier W120s are pre-facelift models and they had one of the WORST ever quality issues (from all points of view: mechanical and fit and finish) from any production M.B.

Later (post-facelift) W210 models -2000 and up- improved a little (not a lot), but the revisions helped.

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A. Rosich
CL 500, 1998
S 500 L, 1998
E 320 T, 1995 [Sadly sold ]