Howdy, John! The bad news is that a divorce would be cheaper (ha ha ha).
The good news is that the D100 is a superb camera. I just returned from my first U/W foray with the D100 (in an Aquatica housing), and I loved the results. The D100 is currently selling for about $1700. Most of the major "name-brand" housings are first rate and each has its disciples. My only two housings have both been Aquatica, which makes superb housings, about lower mid-range on the price scale. Ikelites are the cheapest. I'm not very familiar with Ikelite 35mm housings, but my wife has an Ikelite video housing, and it appears to be very well made.
If you only had one lens to start with, I would recommend the 60mm AF Micro Nikkor. It's a great lens, razor-sharp, and focuses extremely closely. It's good for fish ID (small fish - the larger ones you'll have to get a head shot) and macro/close-up. This lens is a bit under $400 at the big discounters like B&H or Helix, although I understand that's the 60mm has been in short supply lately. Nikon also makes some very inexpensive 50mm lenses. NOTE: The 60mm lens was my workhorse with the N90s. However, due to the cropping effect of the D100, it's effectively a 90mm lens now. For that reason, you might want to consider a 50mm lens, which would be a better fish ID lens and would be cheaper too.
When I got my first housing four years ago, most folks were using fixed focal length lenses underwater. I still have the fixed focal length mindset, but I know that many people now favor zoom lenses. I'll leave it to others to make the case for zooms.
With my old Aquatica/N90s set-up, I used my Nikonos SB-105 strobe. You can use this strobe with the D100, but because of the way the D100 handles TTL, you can't use the SB-105 (or any other current underwater strobe) in TTL mode, only manual. Since I do a lot of macro photography, where - in my experience - TTL works very well, I bought a housing for the Nikon SB80-DX strobe, a (land) strobe developed by Nikon for its DSLR cameras. In the housing, the strobe works well in TTL mode. In any case, don't even consider not using an external strobe - you just won't get the vivid colors and contrast without one.
Regarding the memory card, a NEF/RAW file is about 10 MB, so on a 512 MB card, you would get about 50 shots. That should be enough for a two-tank dive, since you will be deleting bad shots. If you wanted to shoot in the best JPG mode, you would get about 150 shots on that card. The 512 MB cards are very cheap nowadays. After the dives you would need to offload the image files to a PC (I carry a laptop) or one of the little portable hard disks that work without an attached PC.
All of the information above is strictly my own opinion. If you ask ten other people, you'll likely get answers very different from mine. And all ten answers are probably good ones. There is a lot of hardware out there to choose from, and a lot of options on how to mix and match it, as you'll see on this forum.
Okay, let's add up the costs for a "starter" set-up:
Housing plus one flat port: $2000+
D100 camera: $1700
60mm lens or mid-range zoom lens: $400
Strobe plus arm set: $500+
512 MB memory card: $150
With shipping costs, that's about $5000. That's a major investment if you're getting it all at once. Many of us already had the lenses, ports, memory cards, and strobes that we used with film cameras, so it wasn't so much sticker shock to go digital.
Actually, the divorce might be cheaper!