Complete all trophies on bronze until you unlock the last trophy (Nurburgring Eliminator).
Use free download car RUF Spyder 3.8 and do the race on gold or even platinum.
You get 500,000 credits for a platinum win, just repeat as often as you like for megacash to buy any or all cars.
Also you can get around 8,000 kudos for each race so you will make rank 1 a lot faster.