"Collected" can happen:
1. Win a car from some events. On the event details page, you can see bottom left the "Reward" for the race. If there is a car, you can win that car, if finish that event. Each season has 3 or more cars to win.
2. Buy cars from the shop. You need different cars for races, so you have to buy cars for some races. The first - cheapest - 10 car is around 93K CR total, but the next 5 around +83k CR.
You can find a farming method for "fast" EXP points and CR:Solution for Cash For Crashes in Wreckfest