If you mainly want to shoot WA (with a penchant for the 10.5mm) then I would suggest two strobes.
I think its hard to fill an area with the 10.5 with just one strobe. Filters work wonders but to me thats a specialized application that works very well under certain circumstances but not one I would want to be limited by. You can use one strobe if all you want is CFWA, but it would be a shame to further constrain your technique. If you are serious you will eventually get two strobes. If you are going Ikelite (which with your budget it seems to make the most sense) then one strobe is a unnecessary detour since you will then need to get a single sync cord then buy a dual. These aren't cheap at ~90-150 bucks each.
Keep in mind that the 10.5 is really a specialty lens. Its very wide and so it can be hard to approach subjects close enough to get enough subject in your shot. Its also hard to keep extrainious items out of the frame. The 12-24mm is still very wide but more versitile.
With $6K (sounds like a lot of money but actually it isn't) I would keep an eye toward upgrades.
I would go with the D200 as I think the added features are worth it. Relative to the total cost of the system the extra $600 isn't significant and your chances of getting a housing right away are better with the d200. Ikelite is saying November for the housing.
I don't think you can get anything but an Ikelite housing on your budget. 10 bar only has housings for the D70. The D80 isn't out yet.
Here's what I would do:
Nikon D200 w/ 18-70mm lens $1650 (you will need a standard zoom for topside)
Ikelite D200 Housing $1300
Ultralight strobe arms $200
2x DS125s with a smart charger and dual sync cord $1600
8" dome port and shade $400 (this is how they are selling this now)
#5510.22 12-24 mm Body only $150
Tokina 12-24mm lens $500
Bag to carry it all in $200
Now you need a bit more for trinkets like a snap clip to attach to your housing, small screwdrivers to assemble it with extra O rings spare battery, memory cards etc. Woops you are alredy over budget!
Alternately you could substitute the 10.5mm for the 12-24mm and get the D200 body only but if this is your only lens for a camera that you will likely use topside as well that would be a shame. You could at least get by with the 12-24mm. And it really is pretty darn wide.