Worry not folks, in the extreme races, there's a German-Car-Only race where you can dominate all in an M5 (I actually personally used an M3 though).
I'm level 21 A-Spec and 19 B-Spec right now, and have all my licenses except the S License (about 25% through that one).
I have the following cars:
GT-R Spec V ~850hp
Viper ACR ~1030hp
Bugatti Veyron ~1200hp (Top speed reached so far: 265mph)
BMW M5 ~680hp
BMW M3 ~560hp
Ferrari 458 Italia ~680bhp (10,000RPM!!!)
Lexus LFA ~690bhp
Lambo Gallardo LP560-4 ~715hp
Dodge Challenger SRT-8 ~710hp
Dodge Ram 4x4 Quad Cab ~730hp :hihi:
There's a lot more, those are just the fully tuned ones I remember off the top of my head.
The key to getting more money is gaining levels to unlock the upper division stuff, and then race the Championships, as you will not just win money for winning each race but also somewhere around 100K-200K for winning the Championship.
If you get all gold on the last race in the Professional Series, the car you win is the Veyron.
Also, another good way to make a ton of money is to play the "special events." So far I've completed the Kart and Rally (and most of NASCAR). The karting isn't that fun, a lot of flooring it and being very light with the steering, but it gives you good money. A solid 20-50K per race depending on the level. The Rally is a LOT of fun and is probably one of my most favorite parts of the game so far, and you get a TON of money from it. Highly recommend it. The NASCAR is well, NASCAR (so it's really dumb - sorry if I offend anyone here), but it does make you a ton of money too.