Lets go piece by piece
Brazilian proposal will face two serious obstacles
First, South American national currencies are among the most stable in the developing world, with 3 exceptions, Venezuelan,
Argentinian and Brazilian ones, furthermore, Brazil has a tradition of currency mismanagement, the country changed its monetary unit seven times since they dropped the gold standard in 1933, every time because of excesive inflation (that's without counting the times they "rounded up zeros" from the existing currency, i.e. reissuing the notes denomined in thousands as denomided in units [for example, a 10000 note became a 10 one])
From the point of view of the other agents in the region, it may look like they most troubled neighbors are trying to escape forward
Second, they planned to extend the system to the MERCOSUR, whose other two founders (Uruguay and Paraguay) have the most stable currencies of the region (the fifth wheel is Venezuela, they literally prefer WoW gold to their national currency), though historically Brazil and Venezuela have obtained the two minor countries support by the simple expedient of bribing their representatives this method is unlikely to work this time, the Argentinian monetary situation is as infamous in the region as in the rest of the world and would likely cause unsumournable resistance in other countries
The only good selling point would be an easier access by the Spanish speaking countries to Brazilian consumer market, but even this is a moot point, Brazilians pay for their imports in USD. On the other hand...
And worse is that no country is even brave enough to tell the others that they should create one currency in the continent except the post colonial African leaders and not the neo-colonial leaders. African leaders are Afraid. African Radical Leaders should come back and speek for us. Gadafi, Nkwame Nkruma, Leopold Sedar Senghor, should return to African scene.
The SUR is exactly like the supranational currencies championed by these people, a scheme where one government buys international validation and support by subsidizing its economically weaker authoritarian neighbors. Cold war era USA's "development aid" to their supporting dictators with extra steps. I.e. it's not about "strengthen their economy in the future" is about letting Lula bribe his neighbors with more ease and without the USA, EU or China sticking their nose