JShuler:
I agree strongly with lucha54 and most of the subsequent posts. My advice echoes theirs, sorry I don't have much to add:
1. SUBTE. Don't overrate the subte. Yet it's fast. But it's often unpleasant, shuts down at 10:30-11pm, and has fewer stations requiring more walking. I used it everyday while living out in Belgrano and Nuñez and Villa Crespo, but now I live closer in (Botanico area) and strongly prefer the colectivo (buses). In certain areas, e.g. lower San Telmo/Barracas toward Parque Lezama, one has to walk toward "shadier" areas to catch the subte. The colectivos are wonderful.
2. BUENOS AIRES. Spend some hours reading the posts in this site over the last 6 month concerning the allegedly worsening political-economic situation here. You'll see many complaints and arguments about the rising prices, crime, garbage in the streets, corruption, and issues with porteño 'quirks' and the infrastructure. Then plan to visit for a month or two to see for yourself and decide if you really want to stay longer. Also know that many visitors arriving with a decent background/smattering of Spanish classes have big problems with the Rio Platense Castellano spoken here, at least initially.
3. WHERE TO LIVE. There are many "culturally diverse neighborhoods", perhaps not so ethnically diverse. San Telmo, Almagro, Villa Crespo, Boedo come to mind. But you just have to visit them and then decide--do NOT overanalyze nor try to decide these things virtually. Suggest pre-arrange your initial housing for 1-2 weeks using one of the temp agencies (bytargentina, 4rentargentina, welcome2ba). I suggest pick an initial area in Recoleta or Barrio Norte that's close-in and safe. Yes these areas are pricier and more tourist-oriented, perhaps less diverse, and yes, you're obviously a seasoned traveler, but we're only talking 1-2 weeks while you get your "BA legs" and check out your next potential home.
Suerte,
Jim