I've found that to be comfortable in cold boating (think sleet, snow, near freezing rain) I need one more layer under my drysuit than I would have under my gore-tex shell for skiing. This confused me a bit at first since I would go skiing at an air temp of say 15-20, and needed more clothes for rafting at 30-35. But water conducts heat at about 20x the speed of air.
Regardless of whether you choose a dry suit or top/bottom combo a pee zipper makes life a lot better. And drysuits are a lot drier then dry pants/bibs especially if you swim; unless the dry pants/bibs have a folding interface to seal with the dry top. http://kokatat.com/media/pdfs/KokatatBibFold.pdf
Some companies rent out drysuits - I don't know if it would be economical for a grand trip or not. If you buy I strongly recommend Kokatat - their customer service is amazing. I sent in a 7 year old drysuit that I had hundreds of days of use in and they sent me a new one free of charge. I have a fair number of friends with similar stories.
I would also want a neoprene hat/swimcap, and both gloves and booties that have sealed
seams - stopping the transfer of water equals significantly increased warmth. I have had a lot of neoprene gloves and the NRS Reactor gloves are my current go to gloves, the NRS Expedition socks are my first choice there.
An Outdoor Research Snoqualmie Sombrero is fairly awesome for cold and rainy - or a regular Seattle Sombrero that is large enough for a fleece hat underneath. In the rain having a waterproof hat vs a hood is a big upgrade to me. But the Grand doesn't get that much rain so this is a luxury item in my book.
I also second the recommendation for rubber boots - they rock for loading in the morning before you get geared up and are great in the evening for going out to the boats to get food/booze/whatever while keeping your feet dry. I used mine everyday of our trip in late October/early November.
I'm in your neighborhood if you want to get a drink and talk more.