Hello!
We are looking to rent an office of cca 120 in Barcelona. It has to be in the center of the city, possibly in a residential area but on a frequent location. Where should we look? Which are the wealthiest “barrios” in the city? What is the average rent/m2 in these areas?

Check out Alex at Lucas Fox as they have a very good selection and can probably help you out.

I think Barcelona is not a cheap city when It comes to accommodation. The average rent per square meter in Barcelona is 13.70 Euros per month.
What ever you want just Go to Google remember Google is your friend.