Bucky, I would recommend going with something bigger than a fish tank. Your koi are going to grow and the problem will resurface over time that you will need to get a fish out and treat it as well as you will want to add another fish later on. that's all just the nature of the hobby. If all goes well your going to have a spawning at some time. again this QT pond/tank can be used to raise the babies. You want something that can handle any size fish that's in your pond. So I would say look into at least 150-300 gallons. make a cheap DYI filter and keep it going year round. A preform pond will also work, along with rubbermaid stock stanks. Any fish being moved has the potential to cause a outbreak. could be parasite, bacterial, or fungal. When a fish is stressed it can and will happen. Koi, goldfish, catfish, It doesn't matter. ALL fish carry parasites, bacterial infections, and fungal infections. Just as humans carry them too. The key is getting the fish settled down, observing them and allowing them to adjust to about the same water conditions as they are going to live in helps reduce this factor. If one gets sick it's so much easier to treat a smaller container than a larger pond. It's also a lot easier to catch a fish in a smaller tank.
Most serious pond keepers that do use the QT tank recommend to QT for about 3-5 weeks. If you'll read on almost every Online Fish plave they always say they have been in QT a minimum of 3 weeks. That's because most parasites will run a life cycle in about 3 weeks so if a outbreak was going to happen they will have usually been able to see it during that time.