The veggie idea is a carry over from the early days of koi keeping in the west. We now know that are better ponds that water gardens and better filters than gravel and veggies.
Take the plunge...go koi pond system. Your koi will be glad you did.
Inspiration...
japanese ponds
For My Friend Jan T - KoiShack
Brittish ponds
Ponds - Uk Style - KoiShack
koi keeping references
Reference Material On Nishikigoi And Nishikigoi Ponds - KoiShack