Thanks for the info. I should clarify I'm in UK in the West Country - we don't get much colder than 0C most winters. And this is an experiment so I don't want to spend more than a few hundred pounds ideally.
None of the options given will be within that price range....unless, you have access to resources and you can use tools.
In our constantly damp climate, it's not just water from above and around but underfoot that is an issue.
Damp accomodation of any airt is not healthy or comfortable, so you need some form of heat, some form of insulation from the ground, and you need some kind of airflow.
I have friends who live in bender type structures.
These vary from ones made like greenhouses (wonderful soaring curved designs from stuff wombled from all over the country) to simple tarp covered tied hazel rods.
They're all comfortable, out of the weather and peaceful places to sleep.
They all have used pallets for their flooring.
They're available, they're sturdy, they will all keep you out of the mud, give you a base for your living quarters.
They can also be insulated, covered in boards, etc., but you need to then think about just what you are going to use for heat...and how you're going to vent it and still have decent air quality. If your budget is tight you'll not be able to buy a tent stove/pipe/jack and bodging needs care.
Benders can be very comfortable, but you need to think about what you're making, about what resources you can find and use, and about the weather. You really need to think hard about the weather. Wind and rain can shatter your best attempts in no time.
My Dad lived 'wild' on Rannoch Moor in the 1930's. He did so in a army pup tent with a down bag, a bit of oilcloth on the floor, a Tilley lamp and a Tilley stove.
So, it doesn't need to be complicated. He said that long wet days, one after the other after the other after the other, were the very, very worst. Otherwise he was was warm enough, but given the choice (hungry 30's, he was recovering from illness, no NHS, no Social Security) and since money was tight, there wasn't, he'd have gone for a tent big enough to stand up inside, have a chair inside too.
Old folks built box beds from wood or stone and filled them with heather. It's springy, it traps air underneath you too and doesn't invite mould in the damp the way that straw does.
Mind in the depths of Winter we have near 16 hours of dark. That's a long time to try to sleep in the damp and cold, so think too on what else you'll do in the dark.
Honestly, best of luck with it. Interested to hear how you get on with it
M