I would not pick that area to stay in. Av. Rivadavia is acceptable for tourists around the area of Congreso (there are a couple of nice hotels there, but they are not inexpensive) and again around Flores, close to Medrano, and around the Río de Janeiro subway station of the 'A' line which is Flores going into Caballito. The rest of Av. Rivadavia I would avoid at night times especially. Your best best is a short term rental apartment. Even if you pay $400 U$ / week that comes out to about $58 U$ per day, which is cheaper than any hotel that is halfway decent and you have a kitchen, bathroom, and usually internet, local phone, TV and housekeeping. Look at some of the rental sites, ByT Argentina, and a bunch of others (that many people on this site could help with) and could give you recommendations.