We always had to keep a cnadle and heavy sleeping bag in our trucks when I drove in Europe, just in case we broke down many hours from rescue. The candle sucked, mostly provided light in that uninsulated truck cab, but the sleeping bag was sure fire way to keep from freezing to death.
As far as heat loss in cold weather situations, I'd say if you're in the artic you're pretty much screwed, but down south here, there's nothing that a good sleeping bag won't take care of. Some of those things are rated to sub zero temps with only your body providing the heat source to keep it warm. I know my Army cold weather bag kept me warm in freezing conditions to many times to count and I'd rely on it under any weather conditions on this continent. But the good news is they've gotten even better in the last few years. Besides, if it got so cold that you need an external heat source you could always revert to the simplest technology of all......build a fire outside your teardrop and warm up. They've been using that trick for countless centuries. It's low tech but reliable.
Concerning water purification, while there are devices that remove suspended solids from ditch water, the issue of bacteria must be addressed. Boiling in conjunction with chlorine tablets ought to kill anything in your water. There aren't many organisms that can survive chlorine, and I'd bet no water borne bugs can live through a dose of it. Do a google search on Chlorine or Bleach to find out what it kills and you'll be amazed. It's why hospitals still use it in cleaning up around their bacteria infested place. The armies of the modern world have been passing out chlorine tabs to drop into suspect water for many, many years to troops that live in much rougher conditions than a teardrop will take you. They are dirt cheep, store indefinitely, and come in lightweight single use portions. No messy measuring needed.
Why re-invent the wheel? Use Uncle Sugars tried and true military technology researched and paid for with your tax dollars. Sleeping bags and Chlorine tabs are the answers to cold weather survival and potable water.
