Mk48 and harpoon are already in the inventory and already integrated with the proposed combat system.
While Harpoon may be in decline in surface ships, its short comings are less of an issue for a sub. While additional range and stealthiness is always desirable, subs are more likely to use harpoons against land targets. Subs already have a very stealthy munition for surface targets which is hard to intercept, the torpedo. Given the 10 years before IOC of the attack class, its not a huge priority right now. Likely candidates are the aforementioned NSM, LRASM, harpoon II, and perhaps storm shadow/scalp. The US will be making some decisions/development in this area, so there is no reason to rush to a decision before they have.
I doubt we will be building any Collins II hulls. However it looks like we will be refitting 5-6 of the Collins hulls to keep them operational and relevant. There really isn't any issues with Collins hulls, they just need major refit to update systems and components. New Engines, new batteries, new sensors,