Words of advice:
1) You will need 10M+ to get all of the cars. Based on my run, it will clock in at around 12M-13M. xDEEVIUSx puts a number it at 11,920,000 to buy all cars. Remember that money you spend on car upgrades is also more money you need to earn to buy every car, so spend wisely.
2) Only buy cars when you need to or wait until you have reached the level for the 50% discount on your current region's production cars. This will save a lot in the long run.
3) If you are concurrently going for the "All cars from (XXX country)" then some cars in the region will not unlock until you have finished virtually all of the races. You can see what it takes to unlock a particular car by attempting to buy it.
4) After you have won all of the races (after level 50) and you have unlocked/won all of the race cars, you will need to change regions twice to unlock some locked cars from the other regions. It will be 100,000 for the first region change, 500,000 for the second region change. Be sure you have bought all of the cars for your region before moving on, as to change regions a third time is 2,000,000, which is wasted money if you didn't plan correctly. A fourth region change is 10,000,000, which is about as much as you needed to buy all of the cars in the first place.
5) Make good use of the auction house to purchase cars you need for less than their retail price. Be mindful that the production and tuner cars in another region might be 50% less than their market listed price when you are in that region. Also, you will win many / most of the race cars by winning race series, so buying them in the auction house is a waste of money and effort. Theoretically you could buy all of the cars from another region through the auction house, but it is probably more time and effort to hunt all of them down than it is to just make the money and change regions at the right time.
6) If you are short on cash to buy the last of the cars, you can keep running the final Endurance race with a hired driver. Use a 55%-cut driver and set him up with a good car with all of the assists turned off, except keep the AI at easy and leave tire wear and gas off. The top race will net you around 175,000 per run and will finish in ~68 minutes.
For those who doubt this figure, my current lifetime earnings in-game are 13.2M, with 85K in the bank, and 43K from online. I bought and sold fewer than a dozen cars in the auction house, and probably lost money on the deal because I bought cars that I later won in races, and then re-sold them to get the duplicate out of my garage. I followed the method described here, and unlocked the achievement