Because of the PDT rule, traders without 25k are not allowed to day trade using margin. A cash account solves this problem. All transactions clear overnight and your funds are available the next trading day. Unfortunately, cash accounts cannot take spread trades, however, they are perfect for directional trading.
Depending on that, brokerages may ask for a minimum deposit in your account that could be higher thanR$100. But for all intents and purposes, yes, you can start trading withR$100.