Your pond maybe a little too small. I have a 6' X 4' oval pond in my backyard. The depth is about 2 feet.
1. You can buy pond liner kit from Costco, around $200-$300.
2. You need to buy water pump to filter the water and to create fountain. That's about $80.
3. In summer, you need UV light filter to kill/control algae, otherwise, in a hot summer day, your pond turns dark green in just a few of hours. Especially if you want to have some fishes in the pond. Cost is around $150. And you need to plug it in 24/7.
4. Don't even think about keeping fishes in your pond in winter, 2-3 feet of water will freeze all the way to the bottom. You need at least 6 feet deep if you really want to. But with 6 feet of water, pond liner is not enough because of the weight of water...you need concrete.
My solution is to scoop the fishes back and keep them indoor in a fish tank and dump them back in the pond once the temperature is around 15 degree. Try feeders first because they are dirt cheap. Don't spend too much on Koi, as you might just be throwing your money into the water.