To answer your original question: yes, you will have to prove that you are using the money to buy a car. I had to give them a copy of my booking form before they would approve a car loan. If you want to borrow extra, it will have to be a seperate loan at the standard rate.
If someone is heading off to the UK, like a lot of people are, to buy a car there will be no booking form. Same is true for any private sale.