Retired
DLC is available via the main menu in a new option called "EXTRAS". They are played independent of the main game, and their save data is even different than your normal save data. However, getting certain rankings and doing certain things (killing X person, getting Y gun) unlock new recruits and weapons in the main game.
Although I have not tested this, it should be possible to unlock all these things before ever touching the main game, meaning the powerful Ruhm could be used on a New Game run. However, since it is DLC and not the standard game, my 2 cents is that anything used in a game from DLC should make the run count as New Game+, or at the very least, its own category.
So yea, you can play the DLC missions right away, and the stuff you unlock from it is available in the normal game as soon as you unlock it.
Although I have not tested this, it should be possible to unlock all these things before ever touching the main game, meaning the powerful Ruhm could be used on a New Game run. However, since it is DLC and not the standard game, my 2 cents is that anything used in a game from DLC should make the run count as New Game+, or at the very least, its own category.
So yea, you can play the DLC missions right away, and the stuff you unlock from it is available in the normal game as soon as you unlock it.