I can only echo Alejandro's comments.
I rented a one bedroom apartment in Las Caņitas for 3 years from 2003. The rental was done directly with the owner, and I only secured the apartment by offering to pay for 2 years in advance (and also avoid the garantia). The other people wanting to rent this apartment could only offer a garantia from outside BsAs province, and the owner was unwilling to do this.
The contract was initially for 2 years with a yearly increase. For the first year the rental was ARS$430/month, ending up in 2005 at ARS$750/month. Current asking price is ARS$900/month. So this will give you some idea as to the increase in apartment prices in a "hot" neighbourhood.
I remember reading a newspaper article about six months ago that said that rental prices on average in BsAs have increased at least 25%/year over the past 3 years - more in "hot" neighbourhoods.
Among the real estate agents in Las Caņitas, 2 months rent in commision is non-negotiable. The only "cheap" rentals around here are dives of minisucle proportion that go for not less than ARS$600/month.
I think the most important things to consider when deciding which area to look in are safety/security, noise, and maybe access to public transportion and distance to local shopping (since I assume you won't have a car). And air-conditioning ... as you know, summers can be brutal!
When I purchased an apartment 2 years ago, I stayed in Las Caņitas, since I value both the plethora of restaurants nearby, as well as being within walking distance of the Palermo Portal shopping center which includes a Jumbo supermarket and an Easy home improvement store.
Are you planning to rent a semi-furnished apartment? Remember, an unfurnished apartment for example will hot have any light fixtures, just naked bulbs hanging from the ceiling ...
As Alejandro mentioned, I think you best bet is going with a firm that is used to dealing with foreigners, and can guide you, and negotiate with the apartment owner.
Just ask if there are specifics that I might be able to help you with (I'm in my apartment in EZE at the moment ...)
John