Unless this is going to be a very large pond, koi probably aren't the best choice. On average, koi need to be stocked at a couple gallons for each fish. They get huge and need lots of room to move around, thus the need for so much water. They also require deeper ponds (more than 3 feet deep) and longer ponds (at least 10 feet long)....and they jump.
Koi are great for large scale ponds, but most back yard ponds aren't the best bet.
There are still lots of beautiful goldfish that would work though. You can keep it simple and cheap and get a few feeder goldies (plan for 1 goldfish for each 10g of water in the pond) or you can get a bit more exotic with the sarasa comets (long tails and tend to be orange and white.) You could also use shubunkins which can be absolutely stunning and easy to see in a pond setting.
I would avoid the egg shaped goldies, like orandas, moors, ryunkins, telescopes, etc. Their body shapes make them move slower and mean they end up as targets for birds and other animals. They also require more tropical water temps due to their digestive tracts.