Here's what I ended up doing when I lived there for these last 3 months:
- VIVO as my carrier. (O2, Claro, and TIM were pretty awful. I tried switching between them when I was roaming.)
- Prepaid SIM card with no voice plan.
- "VIVO TURBO" prepaid data for 14.99 BRL/week (~65 BRL/month)
- Gets you 1 GB per week, no rollover
4 GB/month for ~$21/month isn't a bad deal. You really don't need voice or SMS in Brazil. Literally everyone uses WhatsApp. For any calls/texts outside of Brazil, you're better off using VoIP (like Skype or Google Voice).
You do not need a CPF (Brazilian tax ID number) for a prepaid plan. You can just go to any VIVO store and show them your passport. In São Paulo, you're likely to be able to find a store with an English-speaking manager.
Tip: After you get your prepaid phone, you can manage the account online with the "Meu Vivo" mobile app, including managing your plan. You can also manage your prepaid account using the Meu Vivo website. Instead of authenticating with a CPF, use "Mobile Connect Log-in" to authenticate via SMS.