You can find used metal housings, ports etc at the classifieds in Wet Pixel. Sometimes these go for about 50% off especially if you are willing to buy the whole system. I have the D7000 with a nauticam port. I really like the nauticam port. Ikelite makes a perfectly good system. But by its nature, it is more unwieldly than a metal housing because it is larger and the controls are not as precise.
You could probably use either the 35 mm lens or the 18-55. You probably will need to get a diopter on the lens to make it focus in the port. Usually the 8" port works better on the margins.
With the DX sensor, the 35 mm lens will look like a 52 mm lens. That would be a good mid range prime lens. But I just don't know how close you could focus with it. I am not keen on that application.
The 18-55 is a decent lens and it has the range to take shots of other divers, fish and so on. So it would be a decent inexpensive choice.
The two most recommended lenses for the Nikon D7000 are the Nikon 60 mm Micro (macro) lens. This lens is superb optically. It shoots true macro so it is good for shots of shrimp, arrow crabs, sea horses etc. It is also good for small to medium sized fish. A problem with this lens is that for 1:1, you have to get pretty close so some more wary subjects are hard to shoot. Also, you really can not use it for supermacro. For supermacro using the Nikon 105 micro is the way to go but it is a more specialized lens.
The other lens of choice is the Tokina 10-17 fisheye lens. This lens has a really close focusing distance so you can use it with a really small port of 4". You can also have a small subject in the foreground and landscape in the background. Also, you can take nice shots of divers with this lens. I discovered that it even works very well on the dive boat. The lens does bend all the lines but on a dive boat with everyone looking a bit goofy bent lines kind of add to the vibe in my way of thinking. Since 17 us the longest this lens shoots, if you want photos of fish, you have to be almost on top of them. Most fish will not tolerate this.
Now I also have the Sigma 17-70 macro. It really is not a true macro lens. It just allows nice close up shooting. The lens has a wide range which makes it quite flexible for everything from divers to close ups to landscapes to large fish. It really shines for large fish meaning angel fish and on up. You can shoot this lens with a 6" port but like most lenses it works better with an 8" port.
Some people like rectilinear lenses like the Nikon 12-24 for taking shots of divers and landscapes. I have not tried one of these yet. They have the advantage of not bending lines. You would use them with a large dome and a diopter filter. I think if you want over/under shots, one of these would work the best. I have also heard that they are well liked for shark photos.
