NONE. Because when a merc start to deteriorate, it will cost you a lot. This will appear in every single class and production year so there will be no discount, except you don't mind it or you will sell it after 5 years.
參考: My uncles have the same C 320 and with different pricetags, interior junks and year. After all those plastics fall apart, the price ended up same so think about it. But, enjoy the ride