WARSAW, Poland — Estonia is gearing up to acquire a missile defense system, participating in broader efforts by the Baltic States to boost investments in expanding their air-defense network. The country’s authorities said they plan to select the supplier by the end of March, earmarking up to €1 billion ($1.2 billion) for the purchase. The Estonian Centre for Defence Investments (ECDI), the institution that implements the country’s arms and military equipment procurements, is doing the requisite