First off, I'm not sure if this thread is in the right area of the forum. From the research I have done the 1998 CL600 is based on the W140 platform.
Anyway, I found a 1998 CL600 with about 50,000 miles. It is in near mint condition and is going for about $35,000.
What are some problems to look for that are common with this model? Also how bad is the maintenance on this car. The car has full records and shows regular maintenance. I have a 1993 Lincoln Mark VIII and on average only 1 or 2 things go wrong or break every 3 months or so. It's generally a weekend car and I only drive it about 4,000 miles a year.
Also what is the difference between the S600 coupe and the CL600? I have heard that the CL600 is sportier but both have V12s making the same power (I think).
Mercedes introduced the CL nomenclature for the 1998 model year (at least in the US). It is still the W140 chassis. Your potential purchase is a rare beast. According to the MBUSA web site, there were only 167 CL600's sold in the US in 1998.
As far as cost of routine maintenance, call your local Mercedes dealer (if that's where you're going to have it serviced) and ask. Don't be surprised to hear a simple oil change is $100-150, and a 60,000 mile maintenance is over $1000. You'll be approaching that one soon. Great incentive for DIY.