Oh, it's very easy.
Ask your girlfriend to help you to do something in the field while driving a tractor. She will scream: No, no, no, I can't, I have never been driving a tractor!?". Explain her with patience, it's very simple and at the same time even attractive - no traffic lights, no traffic jams, no precise turns to the left or to the right, no nervous situations, no bad emotions, no policeman round the corner ... See she will finally agree. Take a camera, make some photos of her on a tractor. Later show her these pics - you'll both have some fun of it. Show these photos also to the friends and neighbours. They'll be amazed telling your girlfriend: "Oh my ... you can do it?!".
And then explain her you may look for the special kind of tractor she might like most of all. Maybe antique (according to your idea which), or better you both decide together. Show pics from TBN, videos from YouTube or from wherever.
And then you will see, she will really help you instead of telling you strictly "no"

(This my post contains some dose of humor, sure you have realized that accordingly)
But, on the other hand it's a real story with me and my wife. I had no problem to select a tractor(s), she completely relys on me in that business. Just now, when we've sold one tractor, she independently asks for another for her.
Look at the pics below (sorry for quality). Would you say my wife is unhappy of something?