No matter how good a game is, or a DLC is, I'd say never buy a game just to play a DLC pack. You're gonna waste the money buying the game just to play DLC you'll have to pay extra for.
There was obviously a reason you traded it in/returned it, so DLC isn't gonna change those reasons, since the main game is going to be exactly the same.
Maybe save your money until they get it patched though. I'd hate for you to buy it again only hit one of the bugs which breaks the main quest (or others).