Just a couple of things to say on this topic.
If you are looking for an 09 plate, your diesel options are the 220cdi or the 320cdi: the 270cdi, of which a lot of club members speak highly, was replaced by the 320cdi in 2005.
For some reason, Mercedes never did the cabriolet with a diesel engine.
I was looking at CLKs in the latter part of last year, initially with the intention of buying a cab, but went for a coupe in the end. The main reason was the engine range, as none of the petrol engines give good economy. The 200k is (IMHO) underpowered and also not very economical. 280 gives adequate performance, but even worse economy. As I wanted the car for everyday use, and we do not get that many sunny days for open top motoring, the coupe seemed a better option: also more available and cheaper to buy.
I have had it for six months now, and do not regret it at all. The car is attractive enough to be noticed, but sufficiently understated so as not to attract negative attention. Ride quality is superb, and considerably better than my previous 211 series e-class. Reliability has been fine, with only an oil leak to worry about: I was told this is fairly common by the garage, but a relatively easy fix. Mileage is 42-46 mpg on a run, with urban rarely dropping below 30mpg even in fairly heavy traffic. It never gets as high as the 50+mpg achieved by some 270cdi drivers, but I am not complaining. Only gripe is the interior build quality is not up to scratch, and there are quite a lot of creaks and rattles from the trim. I have got used to these now, but it really annoyed me when I first got the car.
There are quite a few 320cdi's out there, so you should be able to pick up a nice one, and they are a lot nicer to drive than a 220.
Happy hunting.