Quick answer: The best time to visit Buenos Aires is March–May or September–November (spring/autumn). Expect a mid-range budget of about €55 per day. Visa: Visa-free for most EU/US/UK passports (90 days).
Why this window
BA runs late — dinner at 10, tango at midnight. Four days across Palermo, San Telmo, La Boca and Recoleta, fueled by steak and malbec.