If you can get goldfish from a reputable breeder, go for it! Just keep in mind, they don't ship well. I'd love to have a good breeder within driving distance of my house, but I don't. Sadly I'm reduced to pet store fish.
If you do get a fancy goldfish that grows larger than 10 inches, then an upgrade to a 75 gal would be best. It is just as long as a 55 gal, but it's a little thicker and taller. I'd like to upgrade to one eventually because it would allow me more goldfish ^-^ (and that's always a good thing).
As for weekly PWC, when the goldfish are older, then yes, that's a good idea. Smaller fish won't produce as much ammonia and if you can manage to slip some live plants in there, then you'll only need a once weekly PWC when they are younger. I've also experimented with the idea of adding a sump/refugium for plants. I've seen it done with other goldfish tanks, but I just need to find the time and supplies to make it work in my tank.