IF YOU EARNED THIS CHALLENGE YOU CAN NOW OBTAIN THE CAR DUE TO A RECENT UPDATE!
EDIT: They are aware of the Car not being rewarded, and are looking into it. https://twitter.com/ForzaMotorsport/status/81739106095137587...
EDIT 2: The car will not be obtainable in this Challenge, but they have said it will be made available in a future Challenge.
I did 3 different Championships for a total of 16 races and the Challenge unlocked for me.
These things do NOT
count towards this Challenge:Bucket List Blueprints