1. it's fine to use the ATM, just be aware the cash out limits are somewhat low - I can't remember exactly but maybe $100USD equivalent?
2. taxis are the cheapest - I have always used Taxi Ezeiza in the arrivals hall - they will also accept credit cards and USD. Remis are more expensive. Last I heard Uber was a bit of a contentious situation in BA so I have no personal experience with it.