The best case would be Benteke or Lukaku but they're both very unlikely, maybe even impossible. Dzeko would be kinda expensive and maybe a bit too old, too. To be honest, I have no idea who to get beside them and I don't really care. The few matches Schieber started we showed we don't really need a great striker, anyway, lol
Favre is awesome. He's rated highly in Germany since last season.
Edit: Actually, I think he was rated highly in his Berlin days, too. But they sold his important players, fucked the season up and Favre's reputation kinda dropped. But yeah, it's back up.
Unfortunately Benteke and Lukaku are nowhere near as good a player as Lewandowski. And yeah I doubt Dortmund could afford then tbh. And Benteke already earns more than Lewandowski does right now for example.