Whatever U do: DON´T GET A "GESTOR"
They are intermediaries, who say they´ll handle all the paper-work, and red tape, but many times all they do is SLOW DOWN the process in order to get more money from U. And they even may tell you it´s all over with and done, but there may be really someting unfinished.
Once you find a car you like, even if it´s from an agency or "concesionaria", the ONLY SURE WAY to make sure everything is done 100% is to go with the seller to the DMV corresponding to HIS address, since the file on his car will be in that office. Their is only 1 file for every car, and it should be held at the "Registro del Automotor" nearest to the owner´s residence. Therefore, if you don´t live in the same area as the seller, the file will be moved to the office for your location, after you but it.
The people at the "registro" are the ONLY ones who can tell you if the paper work is OK, and ready for transfer of ownership. Just do whatever they tell you, but if you are not fluent in Spanish you better take some argie who can make sure you don´t get hit with some bribe solicitation or whatever. The process has been streamlined virtually all over, so it should be only an hour or two, at the office to get it all done, assuming the seller has all his papers in order.
Suerte