a couple things to consider....
depth at your location, I would highly consider going below what ever the frost line is.
What ever your pond size ends up being, with koi you will want to turn the pond over at least once and hour through the filter sysytem.
koi need 1000 gallons base line and then at least 350 for each additional adult koi(high end koi keepers prefer 1000 per koi at minimum!).
adult koi and potted plants dont mix, koi very much enjoy rooting and shredding them.
some goldfish get quite large and are gentle on your plants.
if not a true bottom drain at the very least consider a clean out drain for much easier spring and fall cleanings of the garden
here is a list of DYI filters for you to browse
*foot note, no matter what you read all filters need weekly or biweekly cleaning, nothing is matainance free!
and I have heard this will even work up north!