"Would it be safe to buy a 2011 honda element sc, which has 120 000km? It means the owner made approximately 40 000km per year. The car is still young but would the fact it ran a lot be critical for any reason?"
You have that backwards. The high mileage means it was driven a lot on the highway. That's when almost no engine wear takes place. Engine wear occurs when it is still cold right after starting and parts haven't expanded yet to fit right. Low mileage vehicles that saw a lot of short trip driving are the ones to be avoided. Sounds like you found a three-year-old vehicle with much less than three years worth of engine wear.