It's apparently confirmed they'll all be bought from Legendary Motorsport, which, when you look at the images shared yesterday, would make sense.
According to those lovely people who access the vehicles early and make a video of it that boosts the hype to higher levels than R* teasing ever can, the modifications available are more on the sensible side, which I'm completely fine with.
The Turismo and Infernus at least are in Sports Classic. Make of that what you will.
Returning player content will apparently be available to everyone as well, seeing as transferring isn't a thing anymore.