Yes, your characters and levels will transfer as it is saved onto destiny servers and not your Playstation. As for the dark below expansion, as long as it's downloaded on the PS3 you can play it. Btw, Purchases on PS store from you account save, so you can look at your purchase history while logged in and redownload the expansion for free.
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For further clarity, all character info is stored on the Bungie Servers under you PSN account. You can only access the characters you had before if you are using the same PSN account. If you use a different PSN account, it'll think you're someone else, an account is also an identity.