Hey, she does not have to believe in it for you to buy the car. I'm not trying to judge her, but I think from the way you posted this write-up, if she didn't believe you yet, she won't apparently. I have never heard someone talk this way about an M5, even though the E39 M5 is the car I will always love. As I said she doesn't have to believe in it, but believe in the fact that you NEED it to give you satisfaction (no pun intended).
I think you should buy this car, she will most probably understand anyways. For crying out loud, this car is not worth standing out there in the showroom waiting for someone to buy it. It should be pampered and kept in safe hands that could take care of it.
Cheers
