Rumor has it that the S4 will be released around April of this year. And google will have their IO conference in May (a place where Nexus devices are often announced).
So your options could broaden by the time you buy. Or the current devices might be more affordable.
BTW, the S3 is not the only mobile with quad band frequencies. A lot of mobiles are quad band. But the S3 is just a very nice device. Also, like many Android phones, you will be able to charge via Micro-USB - which will be fairly easy to find while traveling should the need arise.