Seeing as the Online Dealership has been changed, I recommend going there to get the Pescarolos and the Audis.
But in terms of what 20 Million Dollar car? I'd go for the Mark IV, seeing as you have the Ferrari already. The Jaguar is a prize car in A-Spec Indy 500 and the Ford GT40 (which is another 20 mil. car) can be won on the same race in B-Spec. You can also get the Red Bull X2010 if you want to add that to your collection or, if you want to not piss all the twenty million away, you can get the Miura.