They are the best sedans in the world for any money.
They cost a bit more to fix than a regular MB, but you can't have everything.
I don't watch the W140 market to closely, but I have seen what I consider very nice S600's go for around $20k, and a lot of beat junk for much less. I'd try to find a one maybe two owner car, with a full service history, never hit, no stories, with under 50k miles on it. Then have a PPI done at a good dealer.
My friends 1995 has been dead nuts reliable since they got it in 2004. But when it was traded in, it was so nice that the Lexus dealer kept it and put it on the lot right in front.
A 9 year old trade in has to be pretty damn nice to be put on the lot in the front row.