First read this:
Far Distant Ships.
WRT buying "off the shelf", one of the factors which drove the purchase of the Upholder class subs is they have a much greater range than any other diesel electric boats, an important consideration given the size of our own coastlines and the deployment distances to get to other parts of the world. Upholders are essentially nuclear subs without the reactors.
This puts us between a rock and a hard place. We can buy subs with considerably shorter ranges, we can try rebuilding the Upholders or we can attempt to design and build our own (a very difficult proposition, and the Australians could tell you, look up Collins class submarines). We would be taking just as big a risk by not having subs as well. Politically, a nuclear boat is simply not in the cards (something to do with the "n" word), and a multi billion dollar program won't go over very well either. We live in
interesting times.