Took 1 year old to Rio last summer and going back this summer at age 2. I do not know how different Buenos Aires is, but here was our experience.
1. We brought carseat and used it on the plane. Otherwise, it sat in our hotel room closet. We were staying somewhere pretty centrally located and when we taxi-ed anywhere, we did not use the seat. I never saw anyone else using carseats in taxis, and we took a bus in from the airport.
2. Most hotels have cribs, but at least in Brazil, when I said "crib" the person I was speaking to did not know what I meant. When I clarified that I wanted a baby bed, they understood. It was a standard crib, that they brought up and got ready for us. DD slept like a champ in it.
3. It was a really good time. People on the street are much more friendly to babies than I have found them to be here. We did run into some issues with night time things just starting later than we were used to (dinner reservations at 8:30, food served at 9, etc.) but that's something that's fairly easy to accommodate if you need to.
Have a great trip!