1. Finish the dlc's story missions to unlock sammy renovations.
2. Buy the first renovation then renovate the upstairs and the pool table.
3. Mission to unlock the car will be available then. Finish the mission.
4. Drive the car around until you reach the requested amounted. Not necessary to be in one single run.
Focus on unlocking the car. It's quite fast and good to drive so it can help you wrap anything else you still have to do.