Your conditions in NZ sound quite similar to conditions in Sydney. I use a Nauticam housing and to me it was worth it, I wouldn't recommend the Canon housings as they are not serviceable and I seem to recall some threads saying they were not that reliable. If I was in your position I'd be leaning towards the Fanatsea housing and theG7X II as out of the box it is better suited to macro. In murky waters wide angle shooting is limited and plagued by back scatter in my experience, I'm assuming NZ conditions are similar to Sydney and there will be plenty of macro subjects. It gives you a better chance of getting some nice shots early on.
Of course if you can scrape the money together the Nauticam housing with the vacuum system is great, that green light gives great peace of mind when taking expensive cameras underwater.
Don't under estimate the importance of ergonomics for your housing, it is often reported that people with great buoyancy suddenly are like brand new divers when they have a camera in hand, having things easy to access and use means that there's less things to think about while framing your subject.
I would agree get a good housing and camera and shoot ambient light for a while, Learn how to colour correct it in post and results are reasonable, then think about adding a strobe. I started with one strobe then later on added a second. I would suggest for strobes be sure to get one with manual capability, a TTL only strobe can be problematic.