Willy's brother Ben sees success as an achievement of personality. To take bold risks and be well-liked are the hallmarks of Ben's version of success.
Ben remarks: "William, when I walked into the jungle, I was seventeen. When I walked out I was twenty-one. And, by God, I was rich!"
Much of Willy's own view of success seems to be drawn directly from his brother or, at least, from his view of Ben. This becomes increasingly clear as the play goes on, as Willy converses with Ben on subjects of success, risk, and achievement. Ben is the example of success for Willy, a romantic figure; a hero.
I would say that success is the driving force for Willy Lohman, but he defines success as an almost impossible ideal. He is so afraid of failure that he is no longer able to get any pleasure out of anything. It's not ambition that drives him--it's fear.