To complete this achivement you need to do all the missions on the white list, achievements and tune all the cars in the game (this only includes the normal progression cars without counting the VIP and group ones)
and I’m taking away from this 100% the exclusive cars, event titles and group rank progression
and what would the reward be? Well, completing 100% would give a title that I have no imagination to think of the name and a very exotic vehicle like a mega vehicle or a tank (I think a good candidate for the exotic vehicle is the M85A5-A Trident Super Heavy Gun Carrier Vehicle)