There is a saying that "you can never stop mending a Mercedes S class" since we are referring to relatively old merc's here this saying is pretty true here because they come with so many bells and whistles that it is fair enough to say this.
First of all these cars were made at a period when Mercedes Benz had it's core values of engineering. And since it was an S class (the flagship model) the car had a lot of features that were well ahead of time.(like ESP,ASR Air suspension).
Now this meant that there is a lot of high tech items that could fail when the car gets older, 20 or more years. for eg:- this car is equipped with suction doors. now a failed air compressor for the Central locking system could set back around 150000 LKR. then you have the ABC which is an air suspension system. now this is a common failure among the W220 these days so you could imagine what to expect with W140's air suspension, this to could be wallet busting.
Since this is an S-class spares in any form are gonna be expensive, and then comes the biggest topic of them all FUEL ECONOMY, As for my knowledge I think close to 80% or more of W140's in sri lanka are petrol (in fact I have never seen a diesel). Now the model range starts with atleast In-6 and goes all the way up-to V12. automatic is standard across the range. so summing it up this won't make a great fuel economy value.
My verdict is I leisurely like the W140, It was one of the last Mercedes cars to be designed by the legendary Bruno Sacco. it has the classic design cues of a proper Mercedes. but then this car is pretty much comparable to an elephant these days because this car like an elephant is very big, striking to look at, and very majestic BUT you wouldn't want to own one.
On top of that if you have not owned a Mercedes before and if this is your first time planning to own one. then for the love of good please don't buy this. as you would be better off if you resorted to cars like the W124,W210,W211 E-class models which are more practical for daily use and is the next closest contender to the S-class in the Mercedes line up.