Good idea,but there are score hacks.
So if a player hacks score,then the player can access Lv3 administration.
Also giving car may make bugs..In my opinion its better to give player temp tuned car if he/she reaches 100 score.

public OnPlayerUpdate(playerid)
{
   if(GetPlayerScore(playerid)<100)
   {
   ShowPlayerDialog(Tunedcars,DIALOG_STYLE_LIST,"tuned cars...blabla");
   }
   return 1;
}

And when he/she reaches 400 score,player must be able to use /tcar whenever needed..