Help me get into UW photography (camera question)

I know this is probably a cliche topic on here, but I thought I'd ask anyways. I'm a longtime land photographer wanting to take the leap into underwater photography. I can't afford a full setup for my dSLR, but I want a good starter rig that I can grow with. Specifically I'd like the following characteristics:

~5 MP minimum

Preferably Ikelite housing

FLEXIBLE setup that I can upgrade part by part

TTL would be nice

video mode for short clips

white balance

raw (is this really a big deal with a moderate-mp camera though?)

And to make it really difficult, I'd love to buy this setup used for cashflow reasons. Ideas or suggestions or something ya wanna sell me? Thanks