A lot of great advice on this thread already.
I'll just reiterate what I see as the highlights.
Definitely get enough r-rating in the pad. The bag isn't enough by itself. You WILL be cold in the warmest bag, if you don't get enough pad. Do at least as much research on warm pads as you do on your warm bag. There is plenty of search data in the archives on winter pad strategies and discussions on which ones are warmest and most comfortable.
Pick a bag that is rated at least 10-15 degrees lower temp than you expect to encounter. And pick it from a maker that is known to be pretty accurate with their ratings. Like Western Mountaineering.
Some makers will rate a bag for much colder temps than it can really work in. It's not uncommon at all. Again, the search is your friend here, and you can glean some info from the archives about which makers rate accurately and which makers don't.
Good light clothing that works as insulation can help a lot, as long as it is DRY.
I recommend full head-to-toe coverage, including a balaclava or even a down hood on the head.
Hot water bottle is great, as long as you have a bottle which you are sure will not leak. Even if it does cool off inside by morning, you'll at least had the majority of a good night's sleep. If it's really cold, and you have to get up to pee in the middle of the night anyway, you might heat up the water and put it back in the hot water bottle again.
And lastly, because most people do not want to bother themselves with a vapor barrier layer, look into vapor barrier layers. They work very well in cold weather, but they require some ability to monitor your heat level so you don't sweat. For you, I expect that would be a welcome "problem". So, look into it. It's a specialty system that most people don't want to bother with, but it does work, especially in the very cold.