With the Dealership mod, you can test and buy vehicles on a wide variety of different cars, motorbikes, boats, vans and off-road vehicles. They're different vehicle dealership stores, each one for a different kind of vehicles (you can buy motorbikes on a motorbike dealer, but if you want a boat you have to go to a boat dealer).
Before buying a vehicle, you can customize the colors and plate, as well as drive it around the place.
You can download the mod here: https://www.gta5-mods.com/scripts/car-dealership