Calypsonick, I would submit the following:
1)You are contemplating a DSLR to improve image quality.
2)If you cut corners in quality, the image suffers.
3)Quality doesn't come cheap.
4)"Affordable" and "housed DSLR" do not belong in the same sentence.
So having mournfully accecpted that you will be spending lots of money, all you need to decide is how much quality you want to buy. Good luck in your choices!
As far as your questions:
Camera/Lenses: Nikon or Canon. Flip a coin. Discussion here is worthy of starting a bar fight. The fact it's so divided tells you something...they're both good.
Housings: Aquatica, great overall, tough to get something done by the factory. Sea & Sea, big, well built, and did I say big? Subal, excellent build & design, expensive. SeaCam, a Subal at twice the price. Nexus, a good "middle of the road" in all respects. SeaLux, excellently built, no frills, great value. Ikelite, fantastic price and service, port optics a limiting factor.
Strobes: Ikelite, a bit larger, very reliable, awesome service. (note: after 20 years, these have never flooded on me!) Sea & Sea, mid-sized in everything: size, performance, price. Inon, haven't had these in the water, but interesting. No comment.
Hope this helps.
All the best, James