The Mosler MT900S, which was the second fastest car in FH4 (after that EVO XX) can be obtained normally. Just buy it in the Autoshow,
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Availability
Can be purchased from the Autoshow for 320,000 CR
Possible Wheelspin or Super Wheelspin prize
Can be purchased or bid on through an [Auction House]
You might want to check the often updated FH5 car list in this forum, I think.