Since you're interested in calculating areas, you could reproject everything to an equal area projection like Albers. There is a continental South American Albers. Another possibility would be the Brazil Polyconic. Depending on what version you're on, there are ways to calculate geodesic-based areas even if the data is in a different coordinate system.
Take a look at Add Geometry Attributes and Calculate Field help topics for information on calculating geodesic-based areas and lengths.
If, for other reasons, you want to stick with UTM, probably use UTM 22S, as you seem to have data in zones 21-23.