Michelin Primacy 3 ST would be a pretty good tyre for your Merc C Class. It offers a great ride quality with superb tractiona nd long tyre life. In case you are looking at a Runflat tyre then go for Pirelli Cinturato P7.
However, if your style of driving is aggressive and you are looking at high performance tyres then go for Hankook Ventus V12 EVO.
BMW X5 comes fitted with run flat type tyres and that's what BMW recommends. In 255/55R18 we have Kumho K51 tyres but it has H(210) speed rating. I will recommend you to go for tyres with V speed rating. Please look for various tyre options from here- http://tyres.cardekho.com/search-tyres. If you don't want to go for run flat tyre then I will suggest you Pirelli Scorpion Verde tyre. However remember to keep emergency inflator or BMW's space saver Spare wheel in the car.