Just some random notes:
Best 6 car races:
Renault Clio Cup
VW Bettle Cup
VW Lupo Cup
All those cars are full race versions with nice default setups, lovely to drive. You can win from 6th with no additional mods, but just barely.
Also the 1000 Miles race at the Nurburgring is a blast. Pick something like a Ginetta, Jag E-Type, Shelby Cobra, MBz 300SL, then start and quit until you get an AI field with the same car as yours in it. If you use N3 tires you will just barely be able to outrun the AI.
Best money makers:
Special Conditions, Rally di Capri Easy. You get the Toyota RSC Rally Raid which you can sell for about $250k.
Even better is European Events, German Touring Car Meisterschaft. You can do this on B-Spec in about 10 minutes. $150k prize money plus a CLK race car which you can sell for about $750k.
The catch is that you need a car from the DTM series to compete. Italian Festival, Schwarzwald League B, Legend of Silver Arrow, Opel Speedster Cup, will all give you a car you can use. Then mod it to Stage 4 turbo or something and you can B-Spec about $5M per hour.