Hi and welcome to wetpixel! Really like your galleries man 
I faced the same dilemma and chose the G10 as well.
I think the WP-DC28 is sufficient for my needs especially if you don't care about wide angle photos that much. As far as I'm aware there are no adapters available for the housing that makes it any wider than 28mm underwater. The only thing available for the DC28 (I think) is a domeport from Fantasea called BigEye that just corrects the magnification caused by water back to 28mm.
Yes you will need close up diopters and strobes. Especially strobes. I have found that they're making a world of difference. You just cannot get the colours right underwater just with natural light, especially for stuff like macro where detail, sharpness and high f-stops are required.
I think a diopter is very necessary as well as it helps you focus with the lens fully extended. The G10 focuses really close as is, but going and bumping your housing against a goby isn't really possible so shooting at distance is must. I'm currently using a diopter I found lying around by just holding it over the lens. I plan on buying a reefnet SubSee when the new version comes out (eventually I'm told
).
The other popular option for diopters is to buy an adapter of some sort (dyron makes one) which fits over the front port of the DC28. These adapters accept 67mm threaded lenses, so you can attach close up lenses like the ones fron Inon. The total cost of this is over that of a SubSee+Adapter for equivalent magnification and that's why I'm waiting for it.
That's another disadvantage of the DC28. You can't add threaded lenses etc right away.
The other (although I don't really count it one) is that you can't connect strobes directly to the hotshoe. They have to be triggered optically via fiber optic cable. This is fine and I'm having no trouble at all with it.
I'm using a Sea and Sea YS-27 DX strobe and I plan to buy the more powerful YS-110a later on to compliment it.
You'll also need arms and a tray. I bought a set (along with the strobe) from Ebay from a guy called Nyxell. His store his here. http://myworld.ebay.com/nyxell/
His prices are fantastic and he ships worldwide.
I've only just started using this setup (w/ strobes and arms) but I'm really happy with the results. You can see some of them from my flickr (link in sig).
I think a G10 setup would be capable of some really cool things.
There are loads of people right here on wetpixel doing some amazing work with it. Here's the latest thread I remember
http://wetpixel.com/forums/index.php?showtopic=32471