Until early this year I used a Nikon D90 with Aquatica hosing with dual Ikelite ds161 strobes then upgraded to the Nikon D7000 and Aquatica housing. As others suggested I too recommend the Tokina 10-17mm and the Nikkor 60mm as they do most of my shooting in my local area. Actually I do most of my shooting with the Tokina 10-17mm as I love shooting wide.
Regarding strobes I like my Ikelites as they are a powerful and have a fast recycle time which is what I want with my style of shooting. When I say my style I mean that i get in really close to large animals like sharks, rays, large turtle and take quick series of shots and normally have the tokian 10-17mm Fe set on 10mm. Of course the Ikelites are heavier and need dedicated battery packs but for me its only an issue with weight resrictions when flying as when I am diving I use bouyancy arms to take the issue of weight out of the setup.
Another option to consider is that the camera is often the most cheapest part of a underwater setup. It might make sense before you spend a few thousand dollars on a housing, ports, extensions rings, gears etc etc that it might make more economical sense to sell your Nikon D90 and put the money towards the current D7000? I beleive that the D90 is still getting decent money when sold second hand and still in demand.
If you go for the upgrade to the D7000 like I have done you can have the option like what I have choosen with my Aquatica AD7000 housing and that is to have both Ikelite bulkhead and optical bulkhead. So you can still use your Ds160 in the mean time and if you want to go with optical fibre and Inon z240's down the track you can.
Gives you some ideas to think about.
Regards Mark