This is a fantastic idea. When my grandmother was dying of ALS, she lost her ability to speak -- so she resorted to writing down all her thoughts, as well as her half of all conversations, in a series of notebooks. And now, nine years after her passing, my family loves those notebooks. They illustrate some of the best things about her -- her humor, her intelligence, her wit, her reference to one in-home nurse who she didn't like as "Caligula" (needless to say, my mom immediately had that nurse replaced) -- and they help keep her spirit and memory alive.
Being able to do this for people who know their end is coming -- to have real, tangible evidence of the person they were before they got sick -- creates such a rich and wonderful portrait of the person for their surviving friends and relatives.
Lastly, I'm so sorry about your Dad. He was clearly a wonderful man and an amazing father!