Take your time do it right. YOU CAN REPLACE A AQUARIUM BUT.
Koi are big they need a big pond. This is what I would do. My p'ond is about 22 to 23 ft round 8ft deep 1800,600 Gals. I would make it about 8'x12' 6'deep in the middle 5' on the sides 2'above ground thats 4'&3 below ground 2'above ground.
A bottom drain very important mid water returns nice and skimmer a must and they go in before the liner. pumps,filters & a UV are a lot cheaper now then in spring. Try to find a koi club near you they can help. See as many ponds you can. PS Koi can live over 200 years.