Hey LS! I am so glad that your dad has 'found his fight' - a good attitude goes a long way when battling this disease. Ok, now for some comments from a female perspective, about
your marriage dilemma. Btw Ziff...ya done good!
A wedding is a happy event to look forward to and it may do wonders for your dad's morale and outlook. Married or not, your gf is going to go through this with you anyway. If she is going to stand by you, your marital status is moot. But waiting until your dad's illness plays out won't hurt anything either. And I'm thinking positive here - that the end result is a transplant. There is going to be a lot going on in the coming months, especially if a transplant IS in your dad's future - caregiving stuff, the meetings, doctors' appointments, hospitalizations, transplant, recovery, etc. You are obviously a wonderful, caring son, and I know you are going to play a large part in your dad's journey to wellness. I don't know how long you and your gf have been together or the details of your relationship - but there's nothing wrong with a long engagement. You could buy a ring, ask her to marry you, and then wait until things are more 'settled' with your dad before you set a date.
Unless you are planning an elopement, a wedding and everything involved with the planning, can be really stressful. Even if the couple has a blissfully perfect relationship, there can be some strain put on it. It's very difficult having two stressful, life-altering events going on at the same time. Something usually gives. Sitting down with your gf and talking it through sounds like a great idea. I know if it were me, I would accept the proposal and then say "but lets wait until things are going better with your dad before we set a date.".
Well there's my $0.02 worth! And keep up that positive attitude - that is what needs to surround your dad. He's very luck to have you in his corner.