I have had my C220 CDI Sport with AMG pack for just over a week now, it had 71000 on the clock, but drives and feels like a new car, I am getting around 44mpg on the motorway doing around 70/80 mph.
The only issue with mine is the orthopedic seat adjustment is not working (which I knew about when I bought the car), a job I am looking forward to working on.
As for power, I think this is fine, I had the C200 W204 CDI (136bhp) before the CLK, let me say there is no comparison between the two, the CLK just wants to go. But there again I am no speed king.
all I would say is try to get one with all the bits on, heated seats, parking aids sat nav etc, otherwise they can feel a little under equipped considering the money you have spent on it, this mistake I made with the C200. Hope this helps.