Some of the W220 weaknesses include, but are not limited to: Air-Matic compressors and front struts, various audio system issues, easily clogged body drains (which can cause big bucks to fix), instrument clusters (MBUSA has extended the warranty, not sure about Canada), and like Doc said, transmission issues.

This doesn't include the routine maintenance and wear and tear items. Owning a W220 probably won't be painless, but you already own a W140, albeit a 6 cyl., so you know what I'm talking about.