The tank needs to be large enough for the koi to be able to turn around and a place where it can retreat for security.
Filtration should be mature sponge filter(s) and the ability to hang mesh bags of activated carbon or zeolite as needed.
The tank should have a proper size heater available.
No lighting is needed, other than when visually inspecting the fish.
Have up to date water test kits available and a product such as Prime on hand.
Have a method to cover the tank to prevent the fish from jumping out.