I'd suggest you go to the local pet or fish store and get a few "feeder" goldfish first, to see how they do in your pond. They're usually about 12c-25c, so you're not risking big dollars there, and you can get white or spotted ones, not just plain orange. And they grow just as large as fancy schmancy comets that cost $4.
When you're ready to spend more money, Sarasas are nice, because they have long flowing tails and fins. And Shubunkins have wonderful spots and speckles all over them.
I do have fantails, including orandas and black moores in my pond, and they have grown and done well. They ARE slow swimmers, though. I have screen covers over the pond, especially the "corral" where these guys are kept.