That's what I'm thinking about doing! I just finished the Roadster endurance with the Miata LS I won, and now I have 100K. Too bad instead of getting the Formula 1, I got another Miata MX-5.
I'm going to buy a Viper, and beat the Trial Mountain and Stupor Speedway Endurances though. Then I'll move on the the Stars & Stripes, N/A and GT World Championship! Everything is going great now!