claiming a disabled daughter as a dependent

No, you do not have to spend more than $14K on her.   As a qualifying child, she cannot provide more than half of her own support.  The parents do not have to provide more than half of her support,  she just cannot provide more than half of it.   Support includes, food, housing, insurance, transportation, medical needs, etc.   Here is a worksheet to determine if she does provided more than half of her own support.  

If your child is totally and permanently disabled, her age does not matter.  She can still be considered a qualifying child if she lives with you for more than 6 months of the year and meets the rest of the criteria to be a qualifying child.  

Gross income is all income in the form of money, property, and services that isn't exempt from tax.   So, nontaxable social security disability would not count towards the test for a qualifying relative.   You have to have provided more than half of her support to claim her as a qualifying relative.   But a qualifying child need only to have not provided more than half of their own financial support for the year.