It's worth noting that as far as I know, there's no real tangible bonus for playing on harder settings either, so if you're having trouble on one setting you won't lose anything bar a bit of cash I think, which you'll have more then enough of later and you can always re-run an event. Similarly you'll get more then enough rep over the course of the game so you don't need to worry about that if you get less of that to.
Also to clarify, I am certain the cars only change when you race in different levels, rookie, pro and all star, not with the actual difficulty you use race to race such as serious for example.
Thanks a lot for the tips, Dragonistic! I am, indeed, playing on easy so far