Hi Vina,
I'm one of the Just You tour managers and I'm actually doing the Argentina/Brazil tour 11 March (so unfortunately won't be on yours). As far as what clothing to take...just pack light-ish things as it should be fairly hot - but can rain too! At Iguassu, you may get wet from spray, so again wear something you don't mind getting wet! And it's a good idea to take a plastic bag to protect your camera.
As for currency, get a little local currency before you go, but as you said, US dollars are the best thing to take with you.There are ATMs, but they will give local currency and not US dollars.
Hope this helps...do have a great time!
Sue Davies