I'm not sure how cold it gets by you. I live north of you in PA and my zone is 6b. My pond can get a substantial amount of ice. I have koi, goldfish and shubunkin. My pond is 3 feet deep. I wouldn't want it any more shallow than that, but you might be safer down south.
If you do get ice coverage, you'll have to keep a small hole open in the ice for gas exchange. You can use a low wattage pond deicer, a pond breather or if your ice is minimal, an air stone might be enough.
Now for the koi speech...many people say they want a koi pond, but don't realize how large they can get and how much waste they produce. You need lots of water and lots of filtration. As a general rule, the first koi would require about a 1,000 gallons of water and maybe 500 for each additional koi. I warn you because I wish someone would have warned me. If I had known, I would have just gotten goldfish and shubunkin. Koi are nice, especially butterfly with their beautiful flowing fins, but you can get shubunkin which also have beautiful flowing fins and nice colors too.
Filters....
I wasted a lot of money on commercially bought filters which required constant maintenance and quite frankly didn't perform well at all. At one point I had two pressure filters, a UV light and my water was green like pea soup all summer. Then I added on a bog filter and got rid of all the other filters and UV light. My water has never been so crystal clear. Read the many bog threads on this site and build your filtration system the right way the first time around.
Liners....
Stay far away from PVC liners. They are low in price, but you will regret it when the liner fails, ask me how I know! PVC liners are terrible. They get brittle in a matter of months and puncture easily. The preferred material is 45 mil. EPDM and a second choice is HDRPE. EPDM is very similar to a tire inner tube. HDRPE is strong, but it's quite stiff and some find it hard to work with. If your patient with your folds, HDRPE can be a good choice and in most cases it doesn't need underlayment. EPDM does require an underlayment.
So, there's a lot to learn. Read through this forum and get an education. Ask any questions you might have. We are a friendly bunch and I for one want to help others learn from my past mistakes.