Usual rules apply: make sure the car has a full Mercedes service history and that it’s in good condition. The out-going car lost around 60% of its value over 60,000 miles/three years, so you could save a sizeable chunk of cash by finding a six-month old example rather than buying new. The new car may depreciate slightly less because it has some improvements over the older model, and the frugal 250 CDI model will be desirable thanks to its ability to sip fuel at 55mpg.
The CLS will be a rare sight in the UK. However, good second-hand models should sell well. Keep your service history up to date and your car should yield a decent return. Silver and black cars will be the easiest to sell, and try to make sure you buy a car with leather, sat nav and steering wheel controls to ensure you get the absolute maximum on resale.