The SLK's diesel engines are strong, but if you often drive at low speeds you may encounter problems with the DPF (diesel particulate filter) which needs regular long runs on the motorway to stay in peek condition. The SLK200 and SLK250 supercharged petrols won't suffer from this issue and won't cost the earth to run, or you could go for the SLK350 which is quick and has a howling soundtrack you'll enjoy when the roof's down.