PD specifically said it would include DLC in the game. It said it wouldn't include microtransactions.
If there is car or track content, only players with the DLC will be able to use it. That means a fracturing of the playerbase, which is something PD hasn't done to this point in GT Sport - it would go against the esports core of the game, with some (in fact most) players unable to use the content or participate in events with the content.
The special prize car for the DLC is a visual update of a car (obviously a Mercedes) that's already in the game, like I already said. This would qualify as a new car in the same way that the Walkenhorst M6 is a different car to the M Power M6, or the Hamilton W08 is a different car to the colour variation model, or the Honda Super Formula is a different car to the Toyota Super Formula, but should not have any difference in performance or eligibility.
Full info on this is out tomorrow, before the update on Thursday.