Hope you're saving your pennies, as this one is going to cost. Make sure you spend OVER 2,000,000 as the description states. You don't want to waste that money.
If you are willing to risk trying this in the Auction House it may count towards
However, as MushaConvoy mentioned in the comments, if this is like
then the Auction House WON'T work.
If the Auction gets verified one way or the other I will update this solution.Alf Ganikiller has verified that buying a car from the Auction House DOES NOT work. So much for hope.
As Spilner pointed out in the comments, if you don't currently own the Lamborghini Centenario you may want to grab one of them from the dealer for the next batch of challenges. Specifically:
The Centenario costs 2,300,000 at the dealer. And with the Festival boss discount it should still be over the requisite limit.
If you already own that you may want to follow DeaconFrost2k's advice and grab the Ford MKII GT40 for the upcoming challenge:
If you also own that, the Koenigsegg One:1 is a really good car. Otherwise just grab something that looks cool to you. Other than the Koenigsegg all of my favorite cars cost way less than 2m so it really comes down to personal preference.