I was also seriously thinking about buying a CL500 (C215?) but eventually the too many and apparently very expensive potential issues put me off.
It seems that the suspension has issues, new shock absobers often required and at a cost of circa £1k each. Rust, as usual with any post 2000 Merc (which is why I will probably defect to BMW soon) and in addition various 'small' things like alternators, starter motors, etc, each at around £500 a pop. Add to that the terrible depreciation -I think they will bottom out at £5, some early 2000 ones already at £6.5k- and it put me right off. Shame, as they're such a beautiful car and should be very comfortable too.
PS: No CL420's in that shape, 420 available only on the C140 chassis