I think that if you chose your own car, some stupid person would buy a 1960s fiat 500, as it is very cheep just to have some money left over, then go and spend the rest on visual mods. When they go to race they wouldn't be able to keep up with 400pp cars effectively breaking the game.
If you were to do something like this, you would need some sort of limitations, say when you first log on to GT7 day one, and a screen pops up with five or six cars to choose, each varying slightly in performance, (say like a Focus RS with higher top speed, R32 Skyline for more acceleration, Silvia S15 for better cornering, etc.)
you could start off with 25,000 credits, then all the cars on the screen could cost 20,000 credits off the bat, giving you some kind of one time beginner discount, then after that all return to their usual 35,000/40,000 price. This would give people 5,000 left over for visual mods/liverys/performance mods/buying a kart. Giving people more freedom at the start of career mode.