I don't know your whole story- meaning I don't know if there's any abuse etc.
If I were you I would keep the letter short, simple and totally postitve. My father died when I was 2 from stomach cancer and I would truly give almost anything for a chance to have a father. I only remember him being taken by ambulance to the hospital, where as my 4 older siblings have clear positive memories. If he's sick, try to keep the past in the past, cause all we we have is this moment. I know your story is different but I encourage you to read a question in the Q&A section titled 'Meeting My Biological Father For The 1st Time' by 'dfisk'. She met her father for the 1st time last night. I know you have an actual history, but I don't want you and your dad to miss out. Try to understan he may be struggling with deep issues too ,as you are. Now's not the time to resolve your whole history. I hope this helps