65 gal is rather small for koi. They get to be 2-3 feet long and you need a minimum of 2-300 gal (1000+l) ponds for them.
For a small pond, you are limited to goldfish, even then, I would stick to small varieties (ie not 1 foot long comets). Winter temp is a bit low for the truly fancy varieties (they like to be in the low 20C at a minimum), however, you can keep common fantails & moors outdoors at those temps.
Shubunkins IMO is the best for your setup. They are single tailed goldfish, so are more hardy then the double tailed fancies. They look like miniature kois, and can be found with fancy body colours & markings just like kois. They just don't get as large as kois so are just right for a small setup.
Depending on your pond setup, you can go with a canister filter as suggested. However, since goldfish are fairly messy, you should be over filtering the tank (ie get one that is rated 2-3 times the tank size - something like 600gph or 2400 l/hr). Most pond people have wet/dry filters. Typically, the water from the pond is pump up to a wet/dry filter, then allowed to flow back into the pond with a waterfall or other decorative water feature. A wet/dry with a water fall will create much more splashing & water/air mix, which will maximize O2 exchange, and that is important in the summer .... as gold fish are cooler water fish & need to have high water O2 content (and O2 content decreases with higher temp).
Because of your high summer temp, you will need to provide shade for your fish ... things like lily pads (real or fake), overhanging rocks .... or an umbrella! Also, since this is outdoors, you might need to have hiding places (overhanging rocks most common) for your fish so they don't become food to outdoor predators. Also, with the high summer temp, there will be a lot of evaporation. You might also consider some kind of auto water topup setup, or you will be filling the pond up daily.