Hi Claudi,
I second the recommendation of Bytargentina. I have rented 5 places from them so far. They deal in short term rentals so you have to pay the gringo price but it is still not so bad and the pics that they provide on the site are reliable. Just be careful because they are always taken in such a way as to make the apartment look bigger than it really is. They also provide the apartment size in square meters. Take a good look at this number. Also, assume that any amenities not listed are non-existent. The advantage of using byt is that they have so many apartments that it is easy for you to pick a neighborhood that you want to stay in and just look in that area. Most other companies will only have a few listings in each area and if you don't like these places then you have to look elswehere. Since you are planning to stay almost a year you might want to go for a 1 or 2 month lease with Byt and as you create your network of contacts here in the city you might get a lead on a place in your initial price range. Deals like that do exist but you have to have your nose to the ground to get them. You usually need some kind of local connection and need to deal directly with an Argentine owner which will definately have its advantages and disadvantages. Be forewarned that Argentine business practices can differ greatly from those of other countries. If there is no problem with the place that you initially choose then you can always extend your contract there after a month or so. Another advantage of using Byt is that you could split up your stay into 3 or 4 sections and rent apartments in different nighborhoods. Every area has its charm and advantages/disadvantages. Think about how rich your experience could be if you ventured out and sampled different areas of the city.