When you make a faction, it's an "unofficial" faction. That means everything is funded by YOU and your faction, not by the server.
If your faction is very successful and people enjoy it, you might become "official". That means your faction is scripted and Alec will most likely, depending on your faction, add vehicles and an HQ for it. Those you do have to pay for, some times at least. But it's not all that expensive considering the benefits of faction vehicles.
Faction vehicles cannot be made your own later on. If the faction is disbanded they are most likely removed from the game.
Did that answer your question? If not, feel free to reply.