G'day mate,
Welcome to WP - great to have a few more aussies kicking around the place.
I'm curious as to how much you'd be spending on the G6 setup. It was certainly a nice camera for its time, but it's getting pretty long in the tooth now. It does have pretty good image quality though and great battery life. You're probably already aware of the limitations in RAW write times, AF speed and shutter lag. Only other thing I'd identify is the lack of live histogram, but you can still access it after the shot - just as you do with a DSLR.
I guess my decision would ultimately depend on what price I could get it for (and whether it includes a new warranty). I'd be wanting the price to be very competitve with similar options around today. If you are getting a ripper price, and are aware of the limitations, then maybe it could be a great deal for you.
You've done some excellent research in terms of what is important in an UW setup and you have identified the key components and accessories. I would definitely set aside some cash for the INON wide converter, but the flash is prob the first thing to get (I assume you meant DS50 btw - I'm not sure the 150 does eTTL...)
Another thought...the housing still seems to run for around US$600, then you've got the strobe, lenses and other accesseries to add... I'd be very tempted to consider some of the 2nd hand deals around on 10D/D100/D70/350D setups. Plenty of people are upgrading at the moment to 30Ds and D200s etc. There is nothing wrong with these older dSLR kits and they will be worlds apart from a G6 kit.
I don't know whether you've got SLR gear at the moment, but once you start adding housings, strobes, arms, lenses etc., the cost difference between the cameras themselves starts to become a pretty small component of overall system costs. Given that you are already dreaming of the dSLR route, you might want to ask yourself how much extra it really is to go that way. Maybe a couple more months saving and you might be there and then you won't have sunk AUD$2k into something you will want to dump pretty fast. Shutter lag sucks...But if you are really looking for a compact setup, then maybe there are other things to factor in.
All in all - a tentative thumbs up, but with several qualifications...
Finally, where in Aus are you? Maybe we could catch up for a dive?
Cheers mate.