Moving to Brazil with a disability

"What are the important aspects you would personally keep in mind during the planning phase (social security benefits, ongoing medical treatment, etc.)? "
I found having a global mail provider was important in order to have an address or all information while you are in the process in moving. It is important to be certain all retirement and benefits are up to date and they have tour contact information. It is a great idea to have an exam before leaving and advising you health care provider of where you are going. In my case we discussed YF vaccine, possible need for malaria medication and a two month supply of medication currently taking. Is it a requirement, no. Is it a good idea, yes.
I know of no support organizations in my city.
As to infrastructure; the majority of public transportation is accessible. Sidewalks do have ramps, but maintenance is lacking. Side walks in downtown are brick with trees, which in many cases  the tree roots have made an unlevel surface around them, reducing area for walking or wheelchairs.
One major issue I notice is the Airlines and airports have roll up steps, not sky walks. I see many if not all people with mobility issue having a great deal of problems embarking and disembarking and the complete bus system to and from airline are full with no one caring (or noticing) some one requires assistance, which may put those on the last bus and possible misconnections'.

I agree with Texanbrazil's advice.  A global mail forwarder takes a lot of the doubt out of getting your mail, and is something any potential expat should consider seriously.  Keeping a bank account in your home country and having your benefits deposited there is a good idea:  you can access your funds from here, without exposing all of your assets to the complications of the Brazilian banking system.
Yellow Fever vaccine is expensive and generally requires a special appointment (in the US, at least), but anyone planning on spending time in Brazil should have it, with your doctor's ok.  The risk may be minor in most places, but the impact of infection can be dire.
Infrastructure accommodations can be fair to poor by Western European and North American standards, so anyone living with a disability should visit the areas they are considering, to determine whether they can handle the conditions.  Foz do Iguaçu is an international tourist destination, where some accommodations might be expected; in Manaus, most public transportation is not accessible, and between the speed of the bus drivers and the poor condition of the roads, rides can be hang-on-for-dear-life experiences.  Sidewalks appear to be the responsibility of property owners, so they are extremely irregular:  the change from house to house, with unexpected steps, sudden dropoffs, and slippery material.  I frequently see wheel chair users brave the street, regardless of traffic, rather than attempt the sidewalks. 
This city, like many Brazilian cities dating from colonial times, was built on hilltops, to capture whatever breezes are available.  That makes for some very steep streets, where sidewalks can and do suddenly turn into long stairways.  Brasília, Recife, and Aracaju are pretty flat; many other cities familiar to me have some pretty hilly areas, often in the nicer neighborhoods.
Stores have special, faster checkout for the elderly, pregnant women, and people with disabilities; they are also allowed to pre-board planes.