I recommend waiting to get this achievement until after you have won the Horizon Finale. I had roughly 2.5 million credits leftover after finishing the finale. Buy some cars for the apprentice/journeyman/master achievements from every division and maybe others that are your favorites. You should be most of the way there. If money is getting kinda tight, simply sort the car list by price and start at the bottom. The cheevo will pop as soon as the 50th car is purchased. Also, don't forget to claim your barn finds as those 10 count towards the Car Lover achievement.
If you just want to boost the achievement, simply buy and sell the cheapest car 50x and it will pop. Thanks to shoxyy for this info!