I would bet that PSN works the same way that XBL works.
With XBL, you can delete the entire game from the cache and hard drive, including all DLC purchases. You can then reinstall or download all your content.
Here's the catch ... you do not start fresh with stats, etc. Also, the DLC purchases are going to be available only to the account / username / gamer tag that purchased / downloaded the content in the first place.
Hope that helps.
Once you bought it you have it. There is no way to delete the things you have purchased. Even if you reset your console, your purchases remain. It's all recorded under your purchase and you can re-download it any time without paying for it again.