Nice size.
You filter/pump choice is what is going to either make it easy for you are not.
This is where you want to invest a little more up front. You are going to want a mechanical system and a biological system. They can be acheived in the same system or as two seperate units. I am a DIY guy but they have systems out there that are plum it, plug it and go.
With a biological filter it will take a few weeks for the benneficial bacteria to get established so you might have to deal with green water at some point. This can be delt with by adding a UV clearifer/sterilizer. This should not be necessary after a couple of years of it running.
With koi I would try to incorporate some type of bottom drain to remove wast from the lowest part of the pond.
Avoid filters that sit on the bottom of the pond. Maintainance nightmare.
On pumps I personally have several pumps going so if one should malfunction you still have something to move some water. You want ot turn the water at least 1 1/2 times per hour. More is always better here, try to get 3x.
If you add any kind of spillway/waterfall make sure there are no areas for water to seep behind the rocks/liner and drain the pond. This could happen fast.
Do NOT put and rock in the bottom of the pond(hard to clean) or have any jaged rocks hanging over the sides or shelves in the pond.(dangerous for the fish)
These are just a few of my experiences and you will see what work for you as you go along. Mine is still very much a work in progress but I have learned from my failures and have also learnesd alot from this forum in the last few months.
Not trying to scare you away with bad things just letting you know what to expect sometimes.