Hello,
I like soloduenos.com; as the name implies, listings are direct from the owners. You avoid the 20% fee that many intermediaries charge. Bytargentina.com is a reputable business with lots of supply, but i hate that they wouldn't let me see an apt before renting. I rented from both. In general, the prices of furnished apts here are maybe 3x that of a similar unfurnished place. But you have little choice, unless you want to share accomodation with someone else. If that's something you're willing to do to save money, then i suggest looking here:
http://buenosaires.en.craigslist.org/roo/ You'll save lots of money that way. I also agree that committing to a longer timeframe could be dangerous. There are lots of things that cannot be seen, smelled or heard in a few pictures.
Regarding ATMs, i've found that at most ATMs that use the Banelco network, I can withdraw 700 pesos at a time, and do it multiple times in a row, until I've met my home bank's daily withdrawal limit (around 2500 pesos). You may pay multiple fees at both banks, but it does work.
A tip: Try to only take "Radio" taxis. They typically have a white or yellow sign on the top of the cab that says Radio Taxi (and maybe the company name as well). In my experience, others are much more likely to try to rip you off, either by fast meter, giving you counterfeit by telling you that your money is already counterfeit and switching it, or by driving the "long" way somewhere.
The radio taxi will have just that, a CB radio, inside. I have never had a bad experience with one , while i have already had two in non-radio taxis, and i have taken maybe 50x more radio taxi rides.
Speaking of counterfeit, good to learn the differences between real money and fake money (esp with 50's and 100's). You'll notice cashiers typically check those bills, because it is an issue here, especially for tourists. Ask a local you trust, they'll be happy to show you what to look for.
Enjoy your stay!