Tomales is a tiny little town. I'm sure there are some B&B's around, but off-hand can't think of any motels/hotels. There is a place, the Valley Ford Hotel, about five miles north in Valley Ford. Never been there, just pass it on rides.
Your best bet, especially if you're just looking for something more like a Holiday Inn than a B&B is to stay in Petaluma. It's only about 15 miles east of Tomales, and if you take Bodega Avenue (which eventually turns into Tomales-Petaluma Rd.) out of town, it's a pretty fast drive to Tomales, probably 20 - 25 minutes or so.
As for weather, it can be foggy/windy when you're on the coastal parts of the route, and warmer when you travel inland. I did it in 2007 and don't remember it being particularly cold though -- I think at the most I had arm/knee warmers. I'd bring those things plus a vest to the start, and determine how much you actually need to take along then.