I would suggest a down bag in a Goretex bivy sac.
This and with the down treated for water repellent and the cover with a DWR finish. These can be added with Nik Wax products but the factory finish is probably better. I like Big Agness brand but have had other good ones from the North Face, Marmot, Kelty and others. Start with at least 800 fill power down as this is where there are very few feathers. Get a light weight shell, when weight is not as much of a concern you can use inner and outer liners. A gore-Tex bivvy is good for really wet conditions but unless it is more of the tent type it will get condensation or frost at below thirty degrees.
Do a search as there are many reviews and comparisons. REI has good info on the bags they carry. If cold I will sleep in the wool long johns and put down jackets, vest whatever is soft between the bag and outer liner. Also get the next size up if there is not too much weight penalty. It will be warmer and more comfortable and the down won't be compressed creating cold spots. Also decide if you like the snug fit of the cats Meow type where you move the entire bag with you when you change positions or if you like enough room to turn inside the bag. I used to use the snug ones but now they feel too confining to me so I get the larger sizes now. I used to sell back packing equipment way back when, so hence the long winded answer.
