I'm sitting on 1.5m credits right now I've done a bit more than half the special events, and a good portion of the licenses (about 1/2 way through iB). I've only done events up to Professional, (done with almost all except GT world championship) and I am level 18.
My Strategy is except for tires I have not bought any upgrade parts at all. I just use the cars I need to use (prize cars, CE cars or whatever although the only CE I've used is the Murcielago on the Lambo race) as is, and use the specific cars for the races as well so I've had to buy a few (just got a 458 Italia, and a Camaro SS), but I basically spend the minimum I can.
That said for example to go through the one Mini race I had to buy a Mini at 28k, win the race for 14k and then sell it for 9k for a net loss of 5k for one stupid race. Stuff like that in this game pisses me off. I haven't sold any other cars except that one (I sold it cause my ex has one and she makes me want to kick a puppy).