Update -- it looks like flights from Buenos Aires to major airports in Brazil for two are $1000+, and I would have to pay $160 for a Brazilian visa. So although traveling from Brazil as opposed to Argentina brings the taxes down to about $550 per ticket, saving us about $950, it's not going to really save us to go there.

Good idea though -- for someone wanting to start from South America without a preference for which country, Brazil would be the way to go!