Well, the game kinda adjusts the races to the currently selected car. not to mention that if the car isn't quite up to snuff, the game will tell you and ask if you want to auto tune to the proper level. I've played through half the game with my G37S. Since I have affinity level over 4, all the upgrade parts are free. I'm guessing the latter seasons will focus on the R1-R2 cars, but most of the time you can pick what ever car you like.

There is a complete car list in the cheats section here on gamefaqs

I have created a brilliant spreadsheet for this. But GAMEFAQS wont let me post it in the FAQS

Absolutely you can't do that. You should look for the kind of race then you go to purchase that car, or purchase car with level lower then improve it if the race by the level. If the race is by type it will be easy for you, but some races it need to be normal.

This is simple answer. So, if you want more details I think you should search at FAQs section.

www.gamefaqs.com/xbox360/997653-forza-motorsport-4/faqs

Go to the "buy cars" section and when you see the manufacturers, click "Y" to bring up all affordable cars. It defaults to all classes being presented from F to R1,but if you press X, you can sort it by almost anything.

For a full list including the price,PI and class go to www.myforza.net/f4/carlist.php?.