you will need a LOT of filtration, but if you set it up right, it can be a matter of every day, or every few days pulling out a cartridge (like a pool filter) and rinsing with the hose....
fish on their own aren't really that bad if you have good water movement and plants, but add the ducks... mess....
also, be ready for your ducks to eat your plants and your fish...
if the fish are large enough they may be safe, and if the plants are established well enough before the ducks are introduced you might be fine, but don't buy anything you're not prepaired to lose or replace....
i would say don't spend $40 on a 5" koi.... a 12" koi would probly be safe, but those are serious money.. at least out here in vegas area...
you could build your pond, let your ducks loose and throw in a couple dozen feeder fish every once in a while... some will survive and grow, some will be lunch...
you can provide hidey holes for the fish and the'll be more likley to survive... somewhere that the fish can get in and away from the ducks, that the ducks can't get into or knock over.. etc....
good luck...