Well, invest or not invest, as far as I have observed and understood it, no one nation have been able to sustain as a serious credible military power simply by purchasing their way to it. It's just not doable over the long term.
By hook or by crook, you would still have needed to develop your own internal indigenous capability. Something like I believe India, Pakistan, Turkey and even Iran have done. Something which would have required sustained investment in manufacturing capability (at least the military component), technology development, economic development etc.
While South Korea I would say have done immensely superbly considering their relatively small size. China, of course, have vaulted into another league by now.