Brand dealerships is just too much work for something that affects the server too little in my opinion.
I'd just teleport them into a different dimension on some test track like Fort Zancudo where you could basically go fucking ham if you wish to. And just add a little text like ** The dealership employee drives the vehicle to a nearby owned test track, and allows Player XY to test out the vehicle on the private property. ** and just give people 2-3 minutes. Also, as a requirement, they should have enough money to buy the vehicle and register it before they're even allowed to test drive it, as real life dealerships do take bank statements and such before allowing someone to test an expensive car.