Avoid the '92 and '93 W140 at all costs. They had tons of bugs in them when they were new and some still suffer with electrical problems, etc...
The 600 is a very expensive car to own and maintain. Ignition leads alone are about $1,200 and the intake must be removed to access them. The V-12 is a durable engine although it does tend to leak oil. Resealing timing covers on one of those engines is about a $4,000 job. Some random control modules are way more expensive than on lower models, even the S500. The 600 (and 500) also have the hydraulic suspension in the rear. When the bottom bushings on the struts dry out, they cause a rubbing sound which gets annyoing and the only fix is to do control arms and new struts, which is also a very expensive proposition. Don't let me scare you here, I love the 600, and came damn close to buying one but unless you can afford to maintain it properly, you won't have a very pleasant ownership experience. By the way, an oil change requires removing the left side air box to access the filter housing and the sump holds 10.7 quarts. Figure on about $85 for an oil change, unless you do it yourself. Also don't forget to factor in the cost of fuel and insurance on these cars. It's mighty high. So if you have the funds to keep one up, I say to go for it. You will love having almost 400 horsepower on tap and being able to dust most any other production car from a standing start.