The 320 V6 is always said to be one of the best and strongest made by MB in more recent times. The 350 that came after it had a few major issues to begin with....320 is a stormer. Go find one and enjoy. As a long term clk owner my primary concern would be corrosion and electrics. I wouldn’t have particular concern around the mechanicals other than the usual checks that apply to any old car. You want to look for evidence of servicing, accident damage etc. But as long as it all looks good and the hood and electrics are working fine, go for it. Good news is that parts are not particularly expensive should you need stuff doing.
I’ve had my 2008 350 CLK for many years and - although I use it very little - I still love it as much as the day I got it. In fact I’ve just been out this evening roof down and it’s an awful lot of smiles for very little cash. They’re ludicrous value now. Buy one and enjoy it.