My husband and I have often mentioned this to each other...how do people with walking challenges get around with the sidewalks in such a sad shape?? The city is not designed for this at all...and as MissVeronica stated the traffic does not give the right of way to the pedestrians...I have seen visual impaired get a tremendous amount of help from the people around them though.
There are so many remise companys and private tour guides...I would think this is available to you..but you really need to line it all up before you make the journey.
Make a list of "all" your needs ..things that you take for granted now that are available to you and then go from there. There might even be travel agents in the states that could set this up for you..I am sure you are not the first disabled person to want to see Buenos Aires.
**here are 2 websites I found...don't know if it will help, but a start maybe...
http://www.disabledtravelersguide.com/
http://www.decthird.com/index_e.php
Hopefully other posters can point you in the right direction...best of luck, I hope you can make the trip and that it will be easy for you!