TOKYO, Dec 16 (Reuters) - Japan on Friday unveiled its biggest military build-up since World War Two with a $320 billion plan that will buy missiles capable of striking China and ready it for sustained conflict, as regional tensions and Russia's Ukraine invasion stoke war fears.The sweeping, five-year plan, once unthinkable in pacifist Japan, will make the country the world's third-biggest military spender after the United States and China, based on current budgets.Too few munitions and a lack of spare parts that ground planes and put other military equipment out of action are the most immediate problems for Japan to tackle, military sources have told Reuters.That splurge will provide work to Japanese military equipment makers such as Mitsubishi Heavy Industries (MHI) (7011.T), which is expected to lead development of three of the longer-range missiles that will be part of Japan's new missile force.Other items on Japan's military shopping list over the next five years include interceptor missiles for ballistic missile defence, attack and reconnaissance drones, satellite communications equipment, Lockheed Martin F-35 stealth fighters, helicopters, submarines, warships and heavy-lift transport jets."