So on the note of purchasing a setup. I have an older (like 7 year old) Nikon D70 DSLR, can I get a housing for something like this?
Yeah, but DSLR housing are systems just like the cameras - you'll need lens ports for different lenses, and most likely new lenses sooner or later. Strobes, strobe arms etc as well. The housing itself could actually be quite cheap second-hand for an older body.
I'd like to point out like the others that it's a lot better to just dive first, and if you want memories take some photos topside. Getting some camera-free water time is smart for many reasons: 1) you'll see a lot more stuff than you'd see with a camera 2) you'll learn to spot and find even more as time passes 3) you might actually discover what you want to shoot in the first place - a newbie with a camera is usually shooting just about everything from the first minute of a dive because everything is new and exciting (not a bad thing necessarily, but usually so...) 4) most likely your diving skills aren't yet there to allow you take the photos you'd like to, and the other side of the coin is obviously the safety aspect - the camera takes a lot of attention easily, so a lot of the "diving stuff" has to happen automatically.
If you're already comfortable with cameras and photography, you could make yourself sort of a program to get you shooting. Pick something you'd like to photograph, learn to spot it underwater, learn to approach it safely and without harming the environment, think about composition etc. After you've mastered that, bring the camera.
For example, nudibranches are an excellent starter subject, because they're pretty much immobile, take some skill to spot, don't really require more than a point and shoot camera, but require you to be able to hover motionless to take a picture. The last point is one that many divers struggle with, and needs to be actively practised to learn - just diving might not cut it.
//LN