Buy a paid car, huh?

Hello wanted to buy a car, but this one is still financed, and the seller does not know exactly how it can run. I would look at cars today and would like to take them with me, of course, 1. because it is a long distance and 2. for the sake of impatience. The seller said I could take the car with me immediately, but of course without a letter. Now I don’t know exactly how to handle it best to solve it safely and quickly. If I take the car with me, make a deposit e and everything is recorded in the purchase contract how it should run, is that then sufficient, or should one rather do it otherwise? Have read many threads but to 99% then unfortunately one leaves only conjectures. MfG