Actually, people aren't always willing to spend $50.00 for a large
Koi. I've seen Craigslist ads with people unable to give
Koi away, so that is something to consider. And
Koi grow really fast, mine more than doubled in size in one year. But this year a heron got him, along with a few
Goldfish.
Also,
Koi can be really messy, so you should consider adding a filter of some kind.
Koi are rather sensitive and don't do well in messy water, and a 320g tank is really small.
I'm sorry to be such a downer, just wanted to give you some things to consider. Good luck!!