After about 6 months of research and about 4 months living with a 1992 600 SEL that had 88,000 miles on it when I purchased it, I would have to say that Andy hit on the major common issues that were present with the early models. Interestingly, the man I bought mine from had the dash cluster replaced under warranty so, like Andy, I also have the digital mileage display. My air conditioning evaporator has not been replaced and the climate control works great (I'm also in Florida). But I know this is an issue that runs about $3,000 to repair. According to a knowlegeable source, mites that are on leaves degrade the metal in the system, causing it to fail. My car resided in a garage day and night, and so that may account for why it's still working. Then again, mine may have already had the newer materials that supposedly keep this phenomenon from occurring.
As Andy also indicated, the right rear lift regulator is particularly susceptible to failure and mine has been replaced.
As for on the road performance, there was an issue with the early W140 front suspensions that made them vibrate--later corrected. So if the one(s) you are considering run smoothly at speed, you should be all right.
Oh, one other issue (at least on the 600s) was that the Z-rated tires would wear out prematurely. There was class action litigation over this and owners (I believe) received vouchers for either $5,000 off a new car or $2,500 in cash. Part of the fix for longevity is to "downgrade" to a lower speed rated tire. With regard to tire mileage, my experience at this point is too limited to have any insight.
Good luck and I hope you find a nice car. The car is a wonderful highway car and I feel that my family is very secure when we are traveling in it.
[This message has been edited by dfndr (edited 07-06-2000).]