Keep in mind the W140 was the epitome of benz engg, and cost about USD 65,000 - 100,000/- when new. All spares are available but at their original price. So keep that in mind too.
e.g. If you somehow find a W140 - S600 sedan, take aside USD 4500/year to maintain it in factory condition. If you find a W140 with the M104 engine it will be cheaper but still require about 1000-2000 USD/year. No doubt they are incredible cars to drive, very controlled, quiet and smooth, yet they can dance on your command - but they require their share. If you do opt for the W140 - do not buy the early ones, they were not fully CAN-BUS and being analog they can become nightmares to sort out as it has kilometers of wiring - and that wiring decays badly too.
The later ones can be interrogated and rectified nicely. All models had the AC evaporator made problem and replacing it requires dismantling the dashboard which is a job that requires an entire samawaar of chai, slim tools and very good work knowledge.