Such revisionist history. He already had Memento, Batman Begins, and The Prestige before 2008. He was already nominated for an oscar for Memento, which put him on the map. The Dark Knight was pretty much the most anticipated movie of 2008 BECAUSE Nolan had made a name for himself by then with Batman Begins pretty much reviving Batman movies.
Now to mention you have it so backwards - to say he got "lucky" with Heath Ledger, when everyone was calling it horrible casting. Ledger was cast because Nolan believed in him when everyone else didn't.