This question is a bit complex. I will say the reliability of a Mercedes Benz is average. Brands like Volkswagen are more reliable and Mercedes Benz is more reliable than brands than the likes of BMW and Audi.
No car or brand is 100% reliable but some are more reliable than others. You should consider how reliable a car can be before purchasing. Do research on the brand and model you wish to purchase. You do not want to purchase a car that will spend more time off the road than on the road.