Completed for 2 weeks on car rental for disabled leisure

Heyho, I’ve once heard that you’re on a foreign vehicle, which you borrow for a certain period of time, even take out an additional insurance! Is that true? More precisely it’s a 9- seater, format Sprinter but from another manufacturer. This we borrowed from a support association of a school for a leisure time, this is also fully insured, but if what happens we would have to pay with own participation and damage compensation for the worse classification of the Versiche If someone knows if and where you can get such an insurance so that we might only have a excess of about 500€? About google etc. I always find the same thing with car rental companies, because this association is not a car rental company, but only during the holiday time gives the bus at self-cost prices does not fit. Thanks already for information and tips, would help me a lot. MfG Humbold