the 80s cup is a good way to start. I didnt particularily have fun with the audi cup...its expensive, I dont like audis, and you dont get that good of a prize...another audi, and in amateur, an S4...if you want some real fun, and you ahve the cash, do the mercedes cup. This may seem really hard to do, but try to get about 300,000 credits. Then you can get a CL600, max it out, and very easily take both beginner and amateur Silver Arrows. For beginner you get a kompressor, good to sell for 50,000...and then for the amateur you get a very nice touring car. Its good enough to take the Japanese, American, European, and German touring classes, which gets you a nice bit of change. Doing that would be a great way to get the money you need to advance in the races.
What I did was to take my first few earnings to get a Mustang Cobra R, maxed it out (I like making things their best) and taking the American Chamionship. If you do the 'save before last race' technique, and keep starting then exiting the last race, you can eventually get the Camaro Race car...its another good way to take a big bunch of those amateur races.
The tree races you mentioned are both in beginner and amateur, so if you do enter one of them, I highly recomend you make sure the car is good enoguh to take both the races (you cant choose in the Audi race, but for the 80s one you get tons of choices - make a good one...or use the miata you probably have and get a new one for amateur...I dont think the miata would be good enough to take amateur)