Sorry that I'm late to this party but sounds like a nice car! All of the above posts offer good advice for the most part, IMO. I owned an S500 w140 for a little while with low miles that had sat quite a while. I'd say that as long as coolant looks really good and all heat/defrost/HVAC functions work properly, you're probably ok. On any older used car, I'm looking for evidence of good maintnance, specifically clean oil and coolant throughout its life.
All functions of the car need to work exactly as intended or the price has to reflect the cost of repair but sounds like you bought it right. That last sentence really applies to any vehicle, especially over-engineered luxury cars. What year is it? The 1996 and newer w140 would be the V2.0 of those, there are actually advantages to each version and condition trumps all else. Generally, the newer ones are better, though.
They are built amazingly well and drive like a dream if all is tight. The double pane windows give you an idea of what you're dealing with here. That was a "one and done" deal for Mercedes, you won't find those on a new $180k S600. My ex-GF had a '99 S420 that I drove a lot when it was new and holy sheiss what a car. Best of luck with it.
