Are there any issues that l should pay attention to when shopping?

Pretty poor. Check consumer reports or JD Power, but MB has been plagued with reliability problems across the board until the last year or so. 03-04 are very bad years. Transmission problems and electronic problems plague the CLKs in particular.
l would not touch a used one any older than 06 (when quality started to improve a bit). Reliability is a big reason why used prices are so low for this car.

H-Charles has Half of my Answer.

and here is the rest Half of my Answer & that is if it isnt a CLK320 or CLK500 or a CLK55 than its Pretty useless because the remaining is the CLK230 which is the most Pointless car in the World l think partly because it isvery heavy & the 2.3 litre engine is USELESS Infact its jus like a Dead horse nailed to ur radiator because the Horses r not even Slightely Close to moving the CLk is very Heavey body around.

Good Luck!

linkin Soldier out.