You should get lots of reponses to your question.

You have enough volume to handle koi.

Most ponders with enough fish to create a substantial amount of waste
try to avoid gravel. It is hard to clean and holds muck
anaerobically. My suggestion: avoid gravel like the plague.

Does your pond have a sloped bottom? If it does, a bottom drain will
serve you well. The koi will stir the muck and a collector, such as a
2" drain in a 5 gal bucket with as many 1/2" holes as you can put in
it, will clean your pond for you very well. You will only have to
clean up leaves once a year or so. (That is how we have st ours up.)
Other pick up and filter options would allow an inline filter to grab
the leaves etc. for you.

People have various filter choices. We went for ample veggie
filters. Slow flow through the roots of growing plants will provide
you with good mechanical filtration, good bacerial surface, and a way
to remove nutrients by plant growth. They are very easy to clean by
drining and flushing...once a year.

We have gone with a submereged filter at the deepest point. We are,
however, set up to put in an inline pump. The forer is very quiet.
Perhaps not as effecient.

Others will have more and different info for you.

Enjoy your new pond.

Jim