In my opinion, keeping Koi in a tank is cruel. There will not be enough room for them so there really isn't an option of "if I ever get a pond". Your tank would have to be huuuuge!
You will always have ammonia issues, whether you over feed or not.
They've also been known to bust glass in tanks...

If you want something that you can keep in a tank, and that looks simislar to Koi, have a look at Comets. They're Goldfish, but they they are manageable in tanks. They even have the Koi patterns and colors, they just don't get as big or strong.