If a CLS2 hasn't had maintenance in 40,000 miles, I'd give it a low price, same as before. However, Jay at Mercland is selling it, and his automobiles are of the highest quality. No one has ever complained about his autos.
Having 120k on these engines is irrelevant if they have been maintained on time, and CLS3 sounds realistic. No more than two services per year, or 11,000 miles, should be recorded in the service book. In my opinion, £8,000 is a fair price for a 2008 CLS500. It would be nice if the driver's seat was cleaned, and not everyone likes wood paneling. The bright inside is an improvement above the standard black. Make sure the electrical components are functioning properly, and inspect the pad thickness and disc brakes for signs of lipping if you decide to examine it. Feel free to negotiate any amount of wear. Those round quad pipes are an aftermarket exhaust system accessory as well.
I also wouldn't disregard CLS1. I am not really concerned about heated seats. Never did my E55 heated seats function. I wasn't missing them.
On this one, the price and mileage are more reasonable.