Definitely a good choice in the plastic stock tank. Ponds can get complex and expensive fast. I've installed some smaller units over the years and I like a simple plan with a frugal advantage. Obviously if the pond is going to be the focal point of your garden it's going to cost a lot of time and money in maintenance. Last time I actually considered a pond a country neighbor in a newer build had a huge plastic preform free for the hauling. It was larger than my fullsize truck, lol. The price couldn't be beat and if it had had wheels it would be mine.
By "feeder goldfish" do you mean pet shop purchases or a specific "other"? Bait shops here sell nice, large goldfish primarily for catfishermen by the approximate pound. There are prettier koi at a higher price but for a pond on a budget money spent on plantings would be a higher priority for me. As for the bait store goldies, they're pretty hardy. A good friend and his wife started adding them to their pond after losses to neighborhood cats, birds, and raccoons. I always laugh when he tells his Blue Heron story.
