I'll likely see how far I can progress without buying any cars. That said, I plan to have at least two cars for most models in the game - one to leave as-stock, and another to modify. It'll likely depend on what sort of modifications we can make to the cars, as I'm hoping we'll see an expansion of what GT6 could offer for a lot of models. That's also before we get into the potential return of the RM system to obtain various made-for-game racing versions. Either way, the individual car that I have for modifications will also be the one to have a custom livery, especially depending on if we get a "skin" system, where we can apply liveries that are officially-crafted. (E.g. the FIAGTC Nations Cup skins for the Red Bull cars, or the various liveries used by the AI in GT League.) I especially plan to use sponsor stickers that correlate to actual parts makers for that car, whether they get name-dropped in the model's flavor text, if I can actually see them on various parts like the brake calipers or seatbelts, or if they're on the default livery.
Also, I'll try to avoid buying Ferraris in Rosso Corsa with very few exceptions. Not only do I feel like there are a lot of available OEM colors for a lot of those cars, but they're also very nice colors, too, so I think Rosso Corsa can take a back seat for a change. I'm especially fond of Azzurro Met. on the 458, myself.
But generally, I hope to obtain at least one of every car in the game, and I'm likely going to have two in most cases like I mentioned earlier. I'm not even sure what exceptions there may be, where I'd only have one - maybe if there are unusually limited changes you can make to it, like how you cannot create a custom livery for the Mercedes-AMG W08 in GT Sport.
I do hope we can actually pick our first car in GT7, too, rather than have it be handed to us like in GT6 and GTS. Even if it means not really buying a car from the dealership (used or not) and instead merely winning it from getting good marks on the license tests, haha!