Our family has a genetic form of frontal lobe dementia, but not Huntingdons, with 50/50 passover. My father developed it 20 years before any other known case in the family at the age of 50.
My mother (not genetically linked to the disease) wanted us to be tested.
I do not want to be tested, or for my children to know until adulthood, when the time is appropriate. I won't hid it, which is what the previous generation did. We didn't find out the history until my Gt aunty, Gt uncle, grandma had died and my father was diagnosed. Suddenly we find that my Gt Gt grandmother and Gt Gt aunt died in an asylum. My Gt grandfather (the genetic link died young of an unrelated disease and the family relocated - which maybe why the story got lost.
Cannot help you with huntingdon's, but there is good advice out there and support.
I firmly believe that my children will perhaps have the advantage of being able to screen embryos (controversial I know) and then eliminate passing on the illness for the future.
I had a brief spell of counselling, which was helpful and will probably need more at a later point. We need to sort out the will situation too. Testing does effect insurance, etc.