If you wanted to, I think you could "beat" the game (by which I mean get to the Horizon Finale and see the credits) with nothing but the basic Supra you can pick at the start, as you can choose which championships to enter and the opponents are automatically scaled to match you (they're selected by car type, and auto-tuned/upgraded to roughly match your car's level, rather than being sorted by performance class like in FH1, so no real need to be close to the top of whatever class you're in).
That'd be kinda boring, though.
So really, just do whatever you find the most fun. Buy new cars, upgrade the ones you already have, whatever takes your fancy. You'll get new cars sooner or later anyway - the first showcase event gifts you one, there's the barn finds, and then there's the chance to get cars every time you level up on the wheelspin. And don't forget to download the free launch car pack (and the Day One pack if that's still available) - that's another bunch of cars you won't have to pay any credits for.
The only place where your car's performance class really matters is online, as it's grouped purely by performance class rather than car type, and you'll see, for example, your A-class Lancia Delta going up against Lamborghini Diablos, etc. So for that, you'll probably want your car to be near the top of it's performance bracket.
As for the drivatar difficulty level, you can switch it at any time, so just do a few races and adjust up or down until you hit the sweet spot. I don't consider myself particularly amazing at racing games, but settled on bumping them up to Highly Skilled (with TCS/STM turned off, ABS and the braking line still on), and find it a good challenge.