Your pond looks great! When all the plants fill in you will be amazed how nice and peaceful it will look.
I started out with a basically empty pond and all my plants came free from local people. Look for local mailing lists where people trade plants, or for your local freecycle group. Because my turtle eats the plants, I actually get new plants for free every 6 months or so.
I know koi are the iconic garden pond fish, but they are very dirty and in a pond that small, your water will be fouled in no time. Also, they will eat and tear up your plants and even eat goldfish. I would stick with goldies and maybe mosquito fish (mosquito fish will keep your goldies from multiplying much.)