As an owner of a 99 S420 W/ 50000 miles. I would look for one in the best condition regardless of the mielage. It may be a better idea to buy one w/ 60000 than one with 30k as many of the bugs have been ironed out by the previous owner. I would not even look at any non-99 models as the evaps go south prematurely. Get a V8 S420 or S500. Try to find an S500 Grand Edition (collector car)
Make sure that you get the following options:
Sunshade, Parkatronics, Heated Seats
and definitely no S420 w/ rear axle control because the shocks are very expensive and don't get one with ADS either.
Before you buy the car, have the dealer do a prepurchase inspection. If the car has been neglected, expect at least $1500 worth of maintenance and wear and tear items. If it needs any repair expect no less than 1000. W140 is a car that if you need to ask how much is repair, you probably can't afford to have one. I have '01 S600 and the W220 chassis is a much cheaper car to own than any W140.