First you have to know why are you buying property. Is completely different if you want to buy as an investment and rent the apartment to tourists for short terms, than if you want to live there because you want to relocate or retire in the city.
In the first case, you have to focus in occupancy that you may expect, price of the property, and the expected revenue (rent price). Size and location will determine them.

 
The area to buy if you are investing is definitively Recoleta. This is the premium location for tourists, some of the very most important tourist attractions of Buenos Aires are located there.  On the other hand if you are buying to live, there are additional beautiful areas you may look, like Palermo Chico, Palermo Soho, Puerto Madero.

 
These areas would be great to live but won’t be the first option tourists will choose when they are visiting for first time. As close to Recoleta’s heart it will be better, also more expensive.

 
Size is an important factor. The rents that give more profit margin are weekly rentals. I know you don’t want to live permanently in a 40 sq meters studio (132 sq feet aprox.), but for tourists visiting 7 days is a great deal.

 
It will be bigger than many hotel rooms, and they pay less, in some case almost 50% less than a 4 stars hotel for something equivalent.

Real estate