Looking at a 2013 CLS 550 with 55k miles... all service has been done at MB dealer. Looking forward to reading some of the feedback.

 

I own a 2016 CLS400 4Matic, purchased a year ago as CPO with 35K miles. Love, love, love it. I’ve never felt more connected with a vehicle. One think to note, which I’m sure you’re aware of, is the maintenance cost. I only take her to the dealership for any services. Very costly upkeep; doesn’t matter to me, but many folks would shudder at paying $1000 for front brakes. As far as I’m concerned, it’s the price you pay for having what I think is the sweetest car on the road

 

I bought a 2017 CLS550 brand new. I have never had a single problem or repair issue other than replace both batteries which is more of a maintenance thing. I have nearly 50,000 miles on it. I live out in the country so it never sees stop and go traffic and since I am in my 70s, I do not push it like I would have as a younger man. Another reason for the good performance and reliability is that it has only seen four or five nights out of the garage. After half a century of owning every kind of car and truck you can think of, I am convinced that the two keys are proper maintenance (no one has ever touched any Mercedes I have ever owned except the dealer) and keeping the car enclosed and out of the elements.

The twin turbo V8 M278 engine is sweet and one of the best Mercedes non-diesel motors ever built. It's bullet proof. They don't build them like this anymore. To address the op's questions directly, although the 400 may be a good car, I can't see it matching the performance of the 550. Also, it just seems to me to make sense that the newer the model vehicle the better, even if it costs a little more.