Usually this is how it goes... if the production of the body style runs like
7 years... it is in your best interest to buy from mid year to last year.
So say E class W210 runs from 1996 to 2002... then it is best to buy 2000 to
2002. However in 2000, there was an issue of running extended oil drain interval with regular dino oil... so for W210... I'd prefer 2001 and 2002 for the V-6 engine... If you like the inline engine, then go for 98-99 I believe... same for diesel...98-99.
Now, you are talking about S class... I don't know much about them so using the general guideline as above and do research online about any particular problem... and have the car inspected by MB dealer or specialist BEFORE you buy it if possible.