i worked a pet store for a few years and worked on my family's landscaping business for their pond projects:
number one, in washington for sure, put new pond fish that are to survive the upcoming winter in your pond in late spring at the latest. August is too late. September/October is really too late. Fish need to slowly be accustumed to changing temperatures, and everyone I know who's had success with all their goldfish/koi surviving the upcoming winter starts with either paying waaaay too much on established outdoor/winter surviving koi or starts cheapo fish in the spring.
buying real-deal koi is not something i would advise unless your 36+" deep and 72"+ feet across.
3-4" Butterfly/longtailed shubunkin are about as pretty/expensive as you should get for your first year, if only becuz of the price. <$10-15.
if you don't mind taking a gamble with more expensive fish, get some koi, but don't spend more than $20 on a <4" fish for your first year.
also, cover your pond with sheer netting of some sorts and get one of those tilting asian bamboo waterfall "crackers". RACCOONS. Herons. coyotes. yes even in the city.