http://answers.ea.com/t5/Need-for-Speed-Rivals/Issue-with-Overwatch-Challenge-livery-unlock/td-p/1877943/page/2
Here (see link above) you can find people who completed 17 OW-Challenges, and people who unlocked the Maserati after OW-Challenge #3.
"have done all 17 Overwatch Challenges"
"Level 50 in Overwatch and all 17 challenges completed"
"I'm on PS4 and have Level 50 on Overwatch. I've done all 17 Challenges."
"but with the "Maserati GT MC Stradale". I unlocked the "Overwatch Challenge #3", finished the race and unlocked the car."
"I have completed all 17 challenges, level 50 Overwatch,"
"after Overwatch challenge 3, the Maserati unlocked"
I can confirm exactly this. I have unlocked the Maserati after OW Challenge #3, and i have completed 17 Challenges in total.
I have been cop level 61 and racer level 61 since 25th November 2013; OW Level 50 since 23rd November 2013. Later I even unlocked the highest available pursuit tech for every single cop car (69 (23 cars x 3) + ever single DLC car included) WITHOUT the Timesaver pack (which i haven't bought at all and will never buy).