Hi Peter,
The more and more we here about that CLK500, the more and more I would run from it.
Has all the typical signs of an unloved car - leftover faults, hidden advisories, tatty wheels, very poor paintwork for a newish high-spec MB, etc. Fair enough if it was a very cheap runaround car (£1-2k max) that people deemed not worth spending out on, but I would say it hasn't had an easy life and perhaps might not make the nicest buy in your price range
Back to this CLK55 - it's a great looking example. Has almost everything going for it - nice late model, sensible mileage, great spec, good colour combo etc.
I think your offer is already very fair - in fact I'd say it could be thought of as quite a good offer as far as the seller is concerned, similar cars priced cheaper aren't exactly 'flying out the door' so to speak.
If you do decide to go for it, check the body over very, very, very thoroughly for rust and previous paintwork. It will almost certainly have some very slight rust appearing in the common places, and if not will probably have had some form of corrective paintwork by now.
Not a deal breaker, but slight blisters that you might miss on a first glance can soon escalate to a state where you'd need to get them sorted. If it's had paintwork, you want to be sure that it was a quality repair
Other than that - good luck with the deal and I hope that you will be posting soon with some very nice pics of it sat on your driveway - I'm envious
Will