Again, for OP: Don’t interrupt! I did it in Forza 5 because I thought I got stuck after a few hours.
I lost all my cars and credits. All I had left was half of the setups and designs.
And the DLC, your gamertag owns those, no matter what console you’re on. You “may” need to re-download them, but they belong to you.
As others have said, FULLY install the game, then relax (probably a couple hours) for the update process and DO NOT attempt to play until the downloads are 100% completed and disappear from the download queue.