You said the magic or dreaded word "Koi". Research Koi ponds as they are typically deeper, include bottom drains, settling chambers, Skimmer, UV and filters that may include anything from vortex systems, bead filters, trickle towers, or combos of these items. I built my 16x14x4 with BD, setting pond, bead filter, Skimmer with UV and 45 EPDM liner for $4000. This includes $1000 for coping rocks, plus landscaping, also the cost of concrete block, cement, rebar, plumbing, plants, pump, ect.

In 6 years my Koi have grown to be between 6-8 lbs. They are huge poop makers, so plan accordingly to save headaches down the road.

Happy ponding.