Lewandowski is going to Bayern next season, has already been agreed with Dortmund and why would you need a striker more than a CB?
No, it's not been agreed with Dortmund. Bayern wanted him now, Dortmund refused to do business with them. Barca could try to agree a deal with them and Lewa. I don't think it's likely, but with Villa leaving, getting a striker is a valid option.
It doesn't impact on the purchase of a defender, they already have a budget for that. As for Golden generation, Xavi and Iniesta started coming through more than ten years ago. We've also had Cesc and Thiago. One thing the club is good at developing is midfielders, and if they can't, well they have the money to buy one.