This will likely be the last achievement you get, as it requires $9,870,000 to buy every body.
The quickest way to earn money is to repeat the final event. However, it does cost 350,000 to start, so I recommend not buying any bodies until you have enough to buy all of them. This will avoid having to repeat other events.
You can win the event by winning every race and drag race, and immediately flipping your car in freestyle to lose. It is very unlikely for any opponent to beat these points(~1 in 17,000) so just double check at the end, and repeat the drag race if needed.
For reference, once I had won every career event, I had just over 5,000,000. It should be noted that I only used specialists that increased income throughout playing the game, so you may end up with less. For each run through of the final event, I could earn around 3,600,000, so that becomes 3,250,000 in profit. It took me two playthroughs of the event after initially doing it to get enough, which is half an hour.