For passangers coming from Europe an onward ticket is required. In my case I purchased a business class ticket for a flight from São Paulo to Buenos Aires, at Malpensa airport, in Italy. Once in Brazil, I had the ticket cancelled by going to the Airline office in São Paulo, and got a refund for the complete price (minus handling charges) via a charge back to my credit card from the Ticket agency in Italy. (Only the agent issuing the ticket can give you a refund.). With the exception of the cancellation of the flight, everything was arranged and done by e-mail, and I had the money back within 2 weeks.
Extending a Visa:
Visa extension varies place to place, in Belem, I had to go the Policia Federal where I was given an internet address where I had to register for the service and with the print out from this site, go to a branch of the Bank of Brazil where I payed the requisite fee. With this receipt I returned to the Police office and there they put the stamp in my passport. The first time I went I was told that I would need to prove financial means before they put the new stamp in my passport. In reality this was skipped, when I returned they didn't need anything. But I was informed that a valid credit card would have sufficed.
In Sao Luis, the internet registration was done by a clerk, and the whole procedure took less than 15 minutes as there is a bank in the same building, and no mention was made of financial means.
There are banks to change money at the arrivals lounge before you clear customs. Also once you have cleared customs and are in the terminal building there are numerous ATM which accept the usual cards and systems. (Visa, Maestro, Cirrus, etc.)
Getting to São Paulo from the International:
Once outside the terminal, turn to the right, and get a ticket at the bus agency (around 25BRL), the buses are between the Arrivals and Departures lounges. The bus to Republica/Paulista is the most common way to get to the center. (Repulica is just up from Se, and there is also metro at Republica to get to any other point.) This bus will also stop at the bigger hotels along its route.
There are tourist information booths both at the arrivals and at the departure lounges and are very helpful.