I am uncertain if i need to claim my 18 year old as disabled. She has started having seizures and heart problems. Not allowed to drive and cant work

So sorry to hear about your child's health issues.   You can still claim an 18 year old as your dependent.  She is still a qualifying child.

Do I get a credit or deduction if my child is disabled?

If your child is older than 12 and you need to pay someone to take care of him while you work, you can say he is disabled and you can still get the childcare credit on your federal return.   Or if he is 19 or older you may still be able to claim him as a qualifying child dependent on your federal return.

Aside from those things, there is no federal tax advantage to saying that your child is disabled.   Your state tax laws might be different.