We have two of
these and two of the swivel version on our cart and we're very happy with them. They might be out of your price range though.
I would be worried about the omni-wheels rolling over terrain and possibly getting caught. Also the omni-wheels are going to collect a lot of hair.
I think your wheel choice will work, just may not be as convenient. You might want to think of a way to add brakes, they make the platform more useful if you ever need to work on it.