I have heard that Mercedes cost a lot to keep running, is this true?

The problem with all top marques is labour charges. Our local Merc dealer charges £70+ an hour. The local Jaguar dealer over £100. A lubrication service for the Merc at those rates costs about £200, with £60 worth of Castrol Edge oil included. (It is the same price in Halfords). A maintenance service costs about £500. The auto service indicator has indicated a service about every 12-15 thousand miles and I do a fair bit of town running. I have not needed any but yes, Merc only ENGINEERED parts are expensive, but no worse than several others. Interestingly silly bits like interior light door switches, jacking point blocks etc are under £4( mine got damaged), so they don't profiteer on simple and plastic bits like some do. Merc had a bad quality patch in the early 2000's, but have now reinstated the quality system they 'economised' on in the late 90's. There are a number of ex dealer Merc specialists about who will save you a packet on labour costs, but make sure they have a diagnostic computer. By the way, the supercharger is a model of simplicity and is extremely reliable. The local dealer's experience is one dodgy allegation of excessive noise in 3 years.

No, If you have a service plan its not really that expensive.

If you go and destroy something in the car, it will be expensive but it will also be more powerful than before.

Thats is not true whatever somebody told you. Even with high milege they can still run for a long time. And my dealer told me that they are good cars and they dont require a lot of maintance.