Just because something is cheap, doesn't mean it's rubbish and just because something is expensive, doesn't mean it'll be great.
Nope - but the exceptions to rules are normally that - exceptions. You generally tend to get pretty much what you pay for in life
Everyone likes a bargain (me included!), but nice things tend to cost more.
The reason that CLK55 is cheap is because it's a 125k miler petrol V8 that needs tidying. It's not a £5k minter being sold for £3k, it's a £3k car being sold for £3k
For just a few hundred quid extra I'd give the other one a look:
Mercedes-Benz CLK 55 2dr Tip Auto Amg Coupe 5.4 2000
Better specced (COMAND, Bose, rear blind, black Nappa without discolouration), correct wheels, lower miles, nice plate
Up to you of course and best of luck