I live on Juan de Borbo: in the summer, there isn't a place that isn't totally crazy full. For example I know that the owner of Maka Maka did so well, he bought two other places opposite, called Fox and Vicino: all of them are always crazy full, and with the benefit that if one is lacking staff they can cross the road to support the other. There is no way a restaurant doesn't work out on Juan de Borbo.
Otherwise the Born area is totally crazy full also. The Gotic quarter (on the other side of Laietana from the Born area), does hold plenty of tourists in the summer but is quieter than the other to places I mentioned. Also Eixample Dreta has a good crowd in the day for the local working population.
The San Pere area is up-and-coming and in (maybe quite a) few years will be teeming like the Born. Maybe a good opportunity medium-long term.
Rents I am not sure about, check Idealista.com maybe. Clearly, the busier the area the more expensive rents are.
Food that does well? Burgers, Tapas, Coffee and Cake type places, Pizza are the biggest hitters. Easy, quick and higher margins. Lebanese and Kin style restaurants also do well. In Gracia area there are plenty on the Verde street has plenty, all full.
Not speaking Spanish won't be a problem in general, but maybe get a good Spanish accountancy that speaks English or something.
Let me know how it goes, we'll come and visit.