Ellie,
You asked about adding strobes to your potential camera purchase.
There are several choices. An easy to use type are the optically triggered ones. Sea and Sea have some, the YS-17 currently offered is similar to the YS-25 I have been using in the past. THe YS-27DX is more powerful and likely a bit more money. Inon sells some good strobes as well, the D-2000 and Z-240. I bought the Z-240 recently and have yet to get it in the ocean. There are others as well. There are no Canon strobes for underwater use.
The optically triggered type are easy to use. They are triggered by the strobe on the camera. Connecting the fiber optics to the OEM Canon or Ikelite housings is fairly easy as well.
So, as you can see, there are several options for later add on strobes. As with most things, the more money you spend, the better strobe you will get. It seems in the underwater photo world, the curve is rather steep. It seems to take quite a bit of cash to move up in equipment.
Hope that answers most of your questions.
Regarding housings, there is considerable debate as to which housing is better. First off, the best one for you is the one you can afford and can use. Canon OEM housings are less money and smaller. Ikelite are larger and cost more but have the ability to change pieces to allow wide angle add-on lenses to be used better. Pantima has really nice housings that cost $1000, more or less and I wish I could justify the cost of one for myself. I have used the Canon OEM housings for several yrs and like them. Some on this site take great photos with them. Others really like the Ike housings and take great pics with them.
The Canon OEM housing is less than $200 so its fairly cheap and easy to use for starting. I like the small size for taking on boating and canoe trips.
I am looking into selling my sea and sea YS-25 so if you are interested, you can send me a PM regarding it but I would encourage you to select your camera and housing first.
Hope that is some help.
Dave