I have a single Giant Kokopu, my tank is 90 x 45 x 39 cm. This is probably on the small side, I would recommend at least a four foot tank.
Equipment wise you will need a filter (obviously) and a chiller. Chillers can be quite pricey but are a must for keeping your tank under 20 degrees in the summertime.
As for acquiring a Kokopu you can either catch one (Be careful to take young individuals from healthy populations) or talk to David Cooper at Mahurangi Technical Institute to buy one, this is how I got mine
Useful information:
https://aquariumworld.nz/databases/2-new-zealand-native/255-Giant-Kokopu
https://aquariumworld.nz/forums/forum/34-new-zealand-natives/