You don't say whether you have any preferences in camera system or housing types. I have a Seacam D700 housing body and a D700 to go with it.
The housing has had several modifications that make it much more user friendly. These include a control that pushes either the preview or the function button on the D700. And provides a port lock. There is also a "clutch" that engages or disengages the manual focus gear depending on whether the camera is set to manual focus or auto-focus. These cost over $2000 to add but made the housing and camera much more functional.
New the housing alone cost over $5000. That is about what I would want for the highly modified housing body and the D700 camera body that includes 2 batteries. The housing currently has a 5-pin flash bulkhead fitting.
I also have a Seacam 45-degree viewfinder, a wide port and a few extension rings that could be included in a "package".
Used alone, the 200mm Micro-nikkor focuses down to only 1:1, the same as most other true macro lenses offered by Nikon, Canon, Sigma, and other lens manufacturers. But compared to a Nikon 105mm, the 200mm would require almost twice the working distance, the water between the front of the housing port and the subject. And too often this extra water distance would result in a less sharp image. The advantage of using this lens with T6 close-up lens is that the water distance is reduced to a workable 8-18" while the maximum magnification is increased to 1.8x life-size, a very significant increase over the1.0x of the lens alone. And does this while maintaining a maximum aperture of about f4 to f5.3 for focusing accuracy and viewfinder brightness.
Here are some new shots taken with the Nikon 200mm Micro-nikkor in combination with the 6T close-up lens.
The first three are of a small lizardfish about the length of my finger. The first shot is taken at the minimum magnification which is about .6x, then a shot at an intermediate magnification, and finally one at maximum magnification which is about 1.8x. These are jpgs made directly from the Nikons raw NEFs with no cropping or adjustments of any sort, not even to the exposure.
Also two shots of a pair of small Doto nudibranchs laying eggs on a hydroid. One shot at near minimum magnification the other at or near maximum magnification. The larger of the two Doto's is about about 8mm in length. What is nice is that the sharpness of the optical combination is good enough that you can see individual eggs in the egg ribbon the nudibranchs have laid.