The trip from northern Brazil
to the South Cone can be long. Between Florianópolis, Brazil
and Uruguay (where I live) there are not many places to stop and the seas can be rough. If you want to stop in Uruguay, think about La Paloma, Punta del Este or Piriápolis (100 ton travelift there). No restrictions on boat
Access to Buenos Aires, Argentina
is complicated by a shallow river with many obstacles (Juanjo?) Mar del Plata is easier.
If you leave during the boreal autumm, you'll arrive in mid/late austral winter. Not a good time to go to Patagonia. Follow Juanjo's advise and get local weather
reports often. Gribs are often wrong for coastal areas, affected by thermal winds in the summer.