I just sort of started over with my existing save, (I didn't delete anything). I ran into somebody last night who had just gotten the game, which sort of made me miss the early stages of this game.
So I saved up 20,000,000 credits so I could buy a bunch of cars to bring my credits down to 20,000 so I could basically "start over" with out actually starting over. Then I restrict myself to using only this money and the car it can buy, and race winnings ect.
The cars to buy for this are: ferrari F10 - 10,000,000/ Peugeot 908 -4,000,000/ Citroen race car -5,600,000/ Toyota FT-86 concept -300,000/ Acura NSX -80,000 (all from the new car dealership, note dont sell or use any)
So I dont use any existing cars or any extras from previous play. Also you wont recieve any car prizes or tickets, but credits will get you through it. you'll also have to keep track of the event you've done so far and what level of car to restrict youself to buying (check the race, it displays the level required).
It's definately a bit more difficult than a normal playthrough.