Did Nauticam never make a housing or has it just disappeared from their website? Seems like Subal is the only housing you can buy new though it looks like Ikelite may sell you a big Perspex box for that model. The Aquatica is marked as discontinued.
Personally the limited housing availability would be a red flag for me, housings certainly seem to have come a long way since the 5D II was released 10 years or so ago. If you are spending all that money......Get the housing you want it's generally at least double the price of the camera and if the housing you want doesn't have a model for your camera, get a different camera. I would be concerned that you may have trouble finding parts for a housing that has been long discontinued, like service kits and O rings.
I would also check the port options for the lenses you are considering shooting with for the housing system you are considering.
I found it very hard to go past what Nauticam offers,, the vacuum system, port lock and ergonomics are well thought out. Have you considered buying a second hand 5D-III or a 6D so you can buy a current Nauticam housing. Dealer support also seems good, with a few dealers selling them.