1) A lot of dives are going to be in depths up to about 70 feet, will a strobe be necassary?
Usually a strobe is not necessary for a compact digicam setup, so long as you use manual white balance and good technique (exception - night photography).
2) How hard would it be to attach a strobe to one of the Canon A's (drifting toward the 570 also)
I know that you can get an Inon strobe for the Canon housing. That would be a good low-cost option - and you can always add it on down the road if you even feel that it would be worth your money. I think that the 570 might be discontinued at this point - so make sure that you can get the housing etc at the same time if you go that route so that you don't get stuck w/ the camera and no housing option.
3) What are the advantages/ disadvantages of the canon housing compared to the iklite housings
The Ikelite housing is a lot more money and totally unnecessary for a compact digi setup. You can't beat the Canon housing for their own camera. And you can definitely get all sorts of lenses and strobes from Inon for it.
4) One of the LDS's here has a couple of sealife d600's still in stock and said they would be willing to cut a deal on kit, is this worth looking into and what would be a good deal on for this kit.
I'd recommend that you stick w/ a nice inexpensive Canon or similar for your first uw photo endeavors. It will be great on land, underwater, is priced very well and is totally tried and true. Canon lenses (even those on a inexpensive digicam) are very very good.
My setup's a Fuji F30 but they haven't made the F30/F31fd for quite some time. For a first setup, you will be very pleased with a Canon system - and it should cost around $300 for the camera and housing together.