You could build the lot, and place the car parks where you want the cars to be. Then play your game, make your sim buy the lot, and have them invite sims to the lot who have the kinds of cars you want. Then simply go into buy mode when the car is parked there, and move it off the car space. Or you could have your own sim do this, by giving them a lot of money with motherlode, and have them drive their own cars there.