I haven't looked at the list (spoilers) but from playing the game I can say I've had no problem whatsoever with money, I've brought a few cars and done some upgrading but as GT11 says you can run many races using just the prize cars and early on you don't even need to upgrade them.
I'm pretty sure I ran the entire of the beginner possibly the first Novice B (I think?) series using bone stock cars, didn't even bother upgrading the tires. I think I've only used Anniversary cars in two races and one of those was for the seasonal, couple those and the prize cars with the added money you get from the side events / licenses and the specials like Goodwood and the Lunar thing and I've found that for me cash and cars is not a problem.
I'm currently sitting on 810k or so with half of National A to finish (have prize cars that should work) and just unlocked International B I think, I have been making sure to get Gold on as much as I can (I drew the line at the stupid cone events and got lucky with the X2010 turd). I'm enjoying the singleplayer stuff though so I don't see any reason to want / have to buy credits with real money, for what it's worth though I haven't had to grind or rerun events for money just to buy a car for the next event or a "must have" upgrade to win an event (yet).