Nope, it's only the engine. If it's anything like the other iD software code releases, you will be able to use the content from the game disc with the engine built from source, or you will be able to extract the content from the demo if you dont have the game.
No. “This source release does not contain any game data, the game data is still covered by the original EULA and must be obeyed as usual.”
You should be able to play a total conversion that does not rely on the original game’s assets, however.