how do i talk the mother inlaw into leting my fiance buy this? lol

my thoughts exactly

And IF you do get the car, don't be the one to argue with your (future) mother in law. That's your wifes place. Don't let youself get in the middle of a disagreement there.
I speak from (bad) experiance on that one.
It sounds like you got a great girl and good mother in law.