There are several ways you can earn cars, through the autoshow, barn finds, wheel spins and DLC. Depending on your luck, on your way to reaching level 150 you should unlock a number of cars in the wheel spin so dont feel you need to buy all 175 cars. Over the course of progressing through the championships alone you will likely earn more than enough credits to buy enough cars to hit 175 but do note that some cars are considerably expensive so stick to buying cheaper cars to hit 175 sooner.