That is an absolutely fantastic San Telmo location!!! Grab that place quick before it's gone. I lived one block from there for a year and it was the best place I lived in BA. You would be six blocks from Line C Independencia station, great for when you want to cross the city. Or you can walk over to Paseo Colón (four blocks away),Bolívar or Defensa (one and two blocks away) to catch a bus to go pretty much anywhere. You are so close to so much awesome neighborhood stuff.
Just so you know--People who discourage others from living in San Telmo are usually people who have never actually lived there, or just thought they knew the neighborhood well but were actually wandering around Monserrat where all the squatters live (calle México, etc.) And yes, technically Piedras and Tacuarí (the first two streets east of 9 de Julio) are part of Constitución and not San Telmo, but that nastiness only really starts once you cross over 9 de Julio and then go further south. I lived over there too, at the Monserrat/Constitución border. If you live in San Telmo you will probably never cross over 9 de Julio to do anything over there. San Telmo is a nightlife destination for many with about a zillion restaurants and bars. So when people talk about the streets being deserted, I just assume they weren't actually in San Telmo, but just thought they were. It is a very tiny barrio and the borders are not obvious.