The OP's original contention was more about methodology than the specifics of what we should procure. Briefly,that we base our initial decision making on what is needed to deal with the threat rather than on what we can afford. I see two major pitfalls to this approach; the "threat" could mean all sorts of things depending on who you ask (anyone feel like tackling the North Korean nuclear programme?) and secondly, whether you're a schoolboy buying an airfix jet-fighter, or a govt buying a real one, is any real-world spending decision taken this way?