If she is vintage enough to want to go on a mission, she will likely not dating for having a second class "eternal companion" you who is a convert or one who cannot be sealed to her for lighter.
I am about to get into a relationship with a doctor. First and most importantly, I see major trust issues in your future if you both think the other is brainwashed.
It seems she'll expect you to become Mormon.
I'm on mobile and it isn't letting me post the URL.