Being new to ponds please be careful in reading too much into posted opinions. It's a good place to start, but do your own research too. When it comes to opinions you get them all, on every possible choice.
Filters and pumps are not dependent on pond size. It's the amount of fish in the pond that determines filter needs and the filters determine pump needs. You can grow Koi in a bathtub given enough filters. However that take a pretty serious understanding of ponds. If you really want Koi in that size pond then it's time to put the shovel down and start researching.
You can also put small Koi in the pond and move them out when they get too big. I see lots of people do this. But mostly they never live long enough to be moved.
I would ask how big do you think Koi get? How big do you think Goldfish get? Most people starting out can tell the difference between a Koi and a Goldfish. So I'd want to make sure it actually is Koi you want.
This appears to be an existing concrete pond that was already on the property, but wasn't in use, and you've cleaned it out? I've seen this many times and read many posts asking how to seal the cracks. Steps 1 through 43 is to try dozens of goops, and sealers, bubble gum, etc... Endless really. Step 44 is to cover the concrete with a liner. I think it's easier to just do step 44 first.
Why was the pond abandoned? Because it was poorly made and leaked. Why make their mistake your problem?
It's easy to fix...take a sedgehammer to the concrete. It'll probably come out pretty easy. Then dig out the pond you really want and put in a liner.
To protect the liner from sharp rocks and such you can put down old carpet first, carpet padding, and there's a bunch of different geo fabrics you can use. Even newspaper is pretty good.