Every man should have at least one V12 in their life. That being said, prepare to spend money. W140's have lots of issues regardless of engine size, and the 600's have extra things that can and usually do need attention. If you can't do the diagnosing or the work yourself, it will be even more costly.
The link below takes you to some of the work I did on my old 600. This list is by no means comprehensive. I decided to start documenting the work after I had already done a lot.
http://v12uberalles.com/articles_and_info.projects_page.htm
1996 and later years are the best ones to look for.