One way you might want to approach this is to decide first of all on what sort of camera you want. This could just be a matter of taste or depend on how much you want to get out of it. Then look for a specific make and check out
http://www.digideep.com/ to see if there are housings available for it.
One thing that you might want to consider when looking at a camera is does it have at least one of, but preferably all of, manual, aperture priority and shutter priority modes. Point and shoot mode rarely gets it right under water. A decent macro mode can be a bonus if not using an SLR/DSLR
If you're comfortable using a camera on land and like to be able to switch lenses then you'll need to look at an SLR/DSLR but this'll push your costs up well above your budget. Taking pictures underwater differs a fair bit from land so you may want to start with something a bit more mid range and later look at wet lenses to mix things about a bit. As Dr Mike said, you'll more than likely upgrade kit after a while so it's best to start with something you won't mind forgetting about later.
The housing prices differ quite a bit. Some camera manufacturers make reasonably priced housings for their cameras - Sony, Olympus, Canon and Nikon to name just a few. Housings by the likes of Sea&Sea, Ikelite, Seacam, etc can often be more expensive than the camera inside but like so many things, you get what you pay for and the quality is often a lot higher.
As regards the strobes get a good one as they'll last. There are different ways to synch them to the housing. Some can be synched using a sync cord that is internally connected to the flash shoe on the camera, others may use a fibre optic cable connected from the outside of the housing to the strobe that synchs when the internal flash fires, and some will sync using slave sensors that again fire the strobe when the internal flash fires. Which option you can choose depends on what the housing will allow and strobe housing compatibility. Cost varies depending on size but if I were you I'd get a decent sized strobe with adjustable power settings so you can cover a better range of subjects