To get different Suits, complete the game one time to get the Blackbird Suit. Complete the game a 2nd time on the same save file to unlock the Papillon Suit.
To unlock Hard Difficulty, complete the game one time using easy or normal difficulty.
Get money to upgrade Suit, play the trial missions over and over to get money.
To unlock new stuff, complete the game one time using Normal difficulty to get an extra conversation between Vanessa and "The Client".