You can skip any career reward cars you can't afford at the time (instead of choosing the credits or driver gear) then you will find them in the progress/rewards section in the main menu for purchase later.
There is a new Bounty Hunter Rivals challenge running at the moment. Beat Turn 10 creative director Dan Greenawalt's time. You get the 2010 Audi TT RS Coupe for setting a time, and if you beat his time you will also get another car. His current time seems quite slow, but he will no doubt be improving it before the event ends.
Speaking of rivals, I am playing quite it quite often at the moment. It's fun driving a few events each time I boot the game up and making sure I am on top of as many friends leaderboards as possible