D200 is a great camera, you will definitely be happy with it.
Lenses, i would start with the 60mm for macro and fish portrait type photos. Then add a WA zoom for some flexibility, say the Sigma 10-22. Or if you want one "do it all" lens i would suggest the Sigma 17-70mm, it has a macro setting so that is good.
The flip down macro is something similar to what Dennis is referring to, a Woody Diopter that fits on the front of the flat port and can be taken on and off underwater. But i wouldn't worry about that quite yet. Start with two lenses and go from there, one macro and one wide.
I would suggest the Inon 240 or Ikelite DS125 for strobe.
Housing: what sort of budget and where do you live? Recommendations will differ if you are in Europe or N. America
And of course... you should really enroll in a photo class so you can learn to use it all! Your local dive shop might have one or you can go on location to any number of places, many pros offer on location workshops in prime areas.