Hi Kathy! I love your idea, but the problem is that the rust isn't just surface rust. The metal is rusting away, and in IMHO the structural integrity of the bottom of that tub is too far gone to hold up on it's own, and water for plants you'll have in there (I assume) will only encourage the rust.
So what comes to my mind is to repair it like one would repair the body of a car.
Get rid of as much surface rust as you can, and remove the metal parts that are nothing but rust. Get a kit, or whatever you'll need in materials. It doesn't have to be pretty bc it won't be seen.
As long as it's good and strong to hold the soil if you pick it up, that's what matters. Then use a Rust-Oleum product that stops rust to prevent any further damage. Then I suggest to drill drainage holes bc you definitely don't want water sitting in there. I usually put a piece of aluminum window screen over the drainage hole of my containers and then as
@eber1140 said, I line my containers with cheap landscape fabric over that. Alternatively, you could coat the bottom with that rubbery stuff they show on TV. I think it comes in regular paint cans and spray cans.
I love your idea of using a planter dollie. Seeing how big the tub is, that will be a back saver!
I hope this helps, and please post photos of your completed fairy garden!
Good luck!