Unfortunately, it's not so simple. Although the iPhone 5S is sold here, for example, the only certified Model is the A1457, as you can see in the this page
, if you understand Portuguese.
It's a page from Anatel, the government agency responsible for cellphone communications in Brazil (something like FCC in the USA, I guess) which shows certified wireless communication devices for use in Brazil.
tells me there are at least 7 models of the iPhone 5S out there, so only one of these will work in Brazil, eventually. Unless Apple certifies all models with Anatel, which I doubt will happen.
Also, If you bring your shiny new cellphone shortly after launch, it probably won't work either, because the iPhone is released here with a few months delay, usually.
So, basically, fuck this shit.