The IC600 looks like an astounding bargain on eBay at $150. Don't get caught up in the megapixel counts. I'd only get the newer camera if you want to shoot video. Instead, get a wide angle lens! Search eBay for a 16mm Nikonos 35mm wide angle converter. I got mine for $100, and it's a heavy piece of glass. The converter has a 58mm thread. With the Intova housings, you should be able to use this lens with a 46-58mm step up ring. Just make sure not to get the Bayonet mount originally designed for Sea & Sea cameras.
Flash- if you plan on getting an external strobe, put a piece of underexposed slide film over the camera flash. It will block the flash's visible light, but still emit infrared to trigger the slave sensor. Inon also makes a "clear photo" product for about $20 that does the same thing. Reef Photo carries them.
If you don't get an external strobe, and want to shoot wide angle... I would get an Auto Magic Filter. They will let you capture colors down to about 10-15 meters. The disadvantage is that you won't be doing any macro during these dives.
If you don't get a strobe, and want to shoot macro photos, I'd by the Intova diffuser.