Yo,
To change money...well change some in the airport when you land, or a better idea is to take the money straight from the ATM and carry no more than 50 with you at any time; if you find something you want to buy over that price, simply go to the bank machine (there are plenty, honestly). Also if and when your stuck with only US in your hand, they do take it pretty much anywhere in S.A. but beware...the rate they give you will suck.
Adapter wise....your looking at
Argentina 220 V 50 Hz, Uruguay is the same.
I'm not sure about American visa's as I'm european and thus they were free when I went...there also pretty laxed about it.
Buenos Aires is kinda sketch at times, but only if you look like an a-typical tourist. Also, its fucking A1 for skating.
Hope that helps,
Peace