My wife did buy a Nikonos-III when she was 20, arund 1980. It is still working perfectly.
I always serviced it myself, replacing the O-rings and lubricating the parts in movement.
At the time we were using it with a flash I did build myself, which did include also an halogen lamp. It was very handy to carry a single compact cylindrical device working both as a powerful torch and as a flashlight.
However, in more recent years we moved to video recording, so I did build a pair of very powerful underwater lights. In their first version I did use dicroic halogen bulbs directly in the eater (to be replaced every year, of course). Later I replaced them with LED bulbs, which are more energy efficient (at 10W each they make the same light as my previous halogen bulbs at 50W each):
Nowadays LED lights are so nice and powerful, that I consider a flash an obsolete technology. A flash has also the nasty effect of making the particles suspended in water more visible. Just use a good LED light and make the photos with steady lighting. The electronics exposure system of the Nikonos-V will ensure to always expose correctly if using a steady light. You will also enjoy the dive more, looking at those wonderful colours which are visible only when an artificial light is available.
I envy your Nikkor 20mm lens. It had always been our dream but it was too expensive at the time. It is considered the best lens for UW photography with a Nikonos. The 15 mm was also excellent, but too much wide angle for many cases. The 20 mm was just perfect, and with less distortion. I would stay with it...
Regarding the film. At the time, after testing many different film types, we ended up using always the Kodak Ektachrome 64. These are diapositives, not prints, but you can always print also a diapo. I do not think that it is available anymore...
But here Wikipedia say that Kodak started making it again in the 35mm format!
Ektachrome - Wikipedia