How reliable is a 2003 / 2005 Benz Cl 500 / 55 AMG?

Not very. Those models are unreliable cars and expensive to fix. I am a big fan of Mercedes-Benz. I am in the MBCA and have owned one or another Benz for about 25 years. However, the decade from approximately 1996 to 2006 was not good for Daimler-Benz - to put it mildly. For a variety of reasons, including expansion of product lines, outsourcing, management issues, and the purchase and later disposal of Chrysler, their product quality suffered greatly. Mostly black circles in Consumer Reports reliability surveys.

I mostly have owned pre-'91 Benzes and own a new one, but any from those problematic years could be a money pit. Why do you think they're so cheap? My 2002 E55 needed a new transmission - it cost over four grand. Price a new air suspension controller and/or front shocks for an S500 and you'll see what I mean. Unless you have a fat breakdown fund in the bank and/or can find a solid, low-mile, one-owner example, I would avoid.

Any USED car is only as reliable as the previous owner(s) made it. If the car was properly cared for and carefully maintained, then the car could be super reliable. If the car was abused and misused by the previous owner(s), then not so much.

Unfortunately, YOU are the one who has to determine if the USED car that you are looking at is going to be reliable FOR YOU. That's why we recommend that before you buy ANY used car (and especially an expensive one), you have a mechanic that YOU trust do a pre-purchase inspection for defects, damage and worn parts, to give you an idea of the current condition of the car.

Be prepared for some very expensive repairs.