Is there a saw mill around you can buy rough cut lumber from? For what you are trying to do, that would be the cheapest route for the framing.
I saw where you said you had to keep it under 200 square feet. A 12' x 16' would be 192 sq.ft.,.
I also have back problems so I know where you are coming from with that. I have built houses etc., for better than 30 years and drawn the blueprints for many of them myself.
They used to build homes with studs on 2 feet on centers, around here any way. They stil do, but they use 2 x6 's for the studs, but for your project, it wouldn't hurt anything to go that route. and they make r-11 and r-13 insulation for 24" o.c.
If you would like, I could draw you a set of plans for a 12 x 16 with a barn type roof so you would have the extra floor space upstairs. I could design it so it would use the lest amount of materials possible. And I wouldn't charge you a dime to do so. I enjoy doing it anyway and I am disabled now for the last two years so it has been a while sense I have had the fun of doing so. I think I still have some paper, either 17" x 24" or 24" x 36" I could use. After I feel I had to sell my blueprint machine

, but I can do a good job on the paper I have.
You said you have 80 acres. Are there any standing timber on them at all. If so, you may be able to find someone with a sawmill to cut your lumber for you. A lot of them will cut on halves, meaning they cut it and get to keep halve of it for doing the cutting. They could cut your framing, flooring, siding, roofing and even wood for some kitchen cabinets.
You also said you had a cord of cedar. That would be enough to cut ceder shakes for the roof. You can buy and old leaf spring from a junk yard from an old truck to make a spliting knife with for the shakes.
If you do have the timber and can find a saw mill owner, All you would have to buy would be a few bags of concrete, a few blocks, the 30 lb. tar paper to go under the shakes, nails and door and windows.
That would be a good way out for you sense you are in the pinch you are in. I know where you are coming from. It took me 8 months before I ever got any disability and I sold nearly everything I had trying to live.
But I wouldn't mind a bit in the world drawing them up for you. I could do it in a night or two.
Let me know if you want me to.
God Bless
Dennis