The disc only contains the original Destiny. All the dlc is download only (hence the codes) so a old second hand disc will be a cheap option. As for characters all the save data for them is stored by Bungie and are not on your system. If you do go for the digital downloaded version delete the game you have (to remove any copyright protection) and download the new version off the store. As long as you use the same account your characters will be waiting.