Manfred,
Always another great adventure, huh?
My wife propels herself around Florida rivers most winters but in a long (7 m), skinny (23 cm) rowing shell. Don't fret about the gators and snakes – they're sluggish in winter and you're really not their preferred menu.
To do miles and miles of flat water, I'd lean heavily towards a canoe rather than a pack raft. If you have no portages, weight and style don't matter a lot. If you do have some portages, then Royalex is much lighter (and quieter) than aluminum. Fiberglass even better and wood or kevlar best, but you won't find that in anyone's rental fleet. If you have portages, it is critical you have a center yoke – all decent canoes do. But some rental fleets have $350 plastic boats from Walmart and while they have 6 cup holders, they have no portage yoke.
What you want is someone like me (friendly, good-looking trail angel with a canoe to loan) 4000 miles to the SE. I can imagine some other plays like finding a canoe on craigslist and offering them $100-200 to delay selling it in order to rent it to you and then sell it a little later. Or maybe someone on BPL knows someone who knows someone.
All else being equal (or close), more wetted waterline lets you cruise faster, straighter and with less paddling input.
If rentals are hard to find or exorbitant, (and you have no portages), you could get the $350 version yourself, strap it on the rental car (BTDT), and leave it at the end with a "It's free" sign on it.
Being who you are (a hell-or-high-water kind of guy), consider a double sea kayak. You'll have to pack tighter and leave the ice chest behind, but you're great at that. The thing a sea kayak allows is to beat upwind or crosswind on days that others have to hole up and hunker down.
My offer of a nice portaging canoe on the Kenai Wilderness Canoe Trails or singles or a triple kayak in Prince William Sound still stands, but I can understand that in December, Michele might want to head south instead of north.