The W220 S-Class was introduced in Germany in late 1998 as a 1999 model. M-B introduced the line in 2000 in the U.S. A. after MANY complaints were issued during the first 1999 year run in Europe.
2000 model year S-class still were plaged with many troubled issues, most of them related to faulty electronics and suspension failures.
2001 mode year improved, but not that much. Most issues were resolved (including pathetic interior plastic quality) until its recent facelift at late 2002.
Conclusion: I would avoid 1999-2000-2001 W220s S-classes and if I am definitely interested in buying one, I'll hold for a 2003 or later model year.
In production runs, usually the BEST years in the life cicle of a car model (especially Mercedes-Benzes) are the LAST two years of production (were most production problems have been resolved).
CL 500, 1998
S 500 L, 1998
E 320 T, 1995