Another thing to look for in purchasing a SDL or SEL is the rear end of the brake support bushings (front suspension). The brake support rods angle back from the lower ball joints (front wheel) towards the center of the car just below the transmission shifter. S class cars are heavy and do eventually wear out suspension parts. The brake support bushings are usally the first to go. The front end will clunk over speed bumps, clunk at initial brake application, and cause wheel vibrations at higher speeds. Your looking at close to $1000 for a full front end overhaul, dubble that number from the dealer.
If your going to spend $10,000, it better be showroom.
The 300SDL is a very nice car. I've got 270,000 miles, my interior is near perfect, with the 6 cylinder it does 30mpg in town, and 34mpg on the road. It starts at first crank, burns no engine oil, and only blows smoke if the fuel filters are due for a change.