Jeff,
I haven’t met anyone in Belize who has an adult child with the type of special needs your daughter has, although I’m sure there must be cases like yours in Belize. Belize is a country that is liberal in regards to how people live their lives. Even when they do have regulations, they seldom enforce many of them. So I doubt they have an “requirements” related to guardians of special needs children. So you will need to confer with a Belizean lawyer.
The laws in Belize are based on British Commonwealth law. They are similar to American laws, but not at all the same. American lawyers are not qualified to provide advice regarding Belize laws. We have a list of lawyers in the Belize Manual’s Rolodex. The better lawyers all have websites. So you should be able to contact them via e-mail, then set up an initial call. They usually will talk to you, for free, the first time. You may be able to get enough info on a preliminary call to determine if you need a lawyer. But I would recommend that you have a lawyer prepare a will and guardianship document to address your child’s care if you plan to live in Belize full-time and he or she will be living with you.