https://www.omnicalculator.com/construction/heat-loss
I was looking at the calculator you listed and I would just be guessing on all the values anyway because I don't know what I have for insulation. I need to get everything right so when I do it i can be happy in the long run.
Yeah, you really need to get some accurate hard data in order to advance properly. The calculator link I provided will get you there if you do your due diligence. It may seem difficult but from reading your posts in this thread, I am confident you would be up to the task.
Like I said "It is time consuming", but well worth your effort. At your outer wall openings (doors and windows) it is easy to determine the thickness of the walls, (2x4, 2x6 and such). The 2x4 wall should be close to 5-1/2" because of exterior sheathing and drywall interior. The year of construction will also help in determining R-values, that is if building permits were obtained. But if that is uncertain, accessing a wall here and there is worth the effort, such as peel back the exterior siding a bit or looking around the edges of an electrical wall outlet. Insulation R-values can be determined with a little creativity on your part.
I see from the product brochure that your current heating instrument is rated at 125,000 btu's per hour, probably overstated. The conventional outdoor wood heaters are actually rather pathetic at actual efficiency. If in fact it is as high as 50% efficient, you see the value is more in the range of 65,000 btu's per hour. Considerably undersized for your demands. Gasification boilers running in their sweet spot can be over 70% efficient.
The benefits of using a gasification boiler with thermal storage are big. When a burn takes place, it goes full bore with little if any smoldering time. Think of the storage unit like a battery, charge it till full and then use the battery to exhaustion, or close to. Burning green or overly moist firewood wastes some of its energy removing the moisture in the wood. The moisture must be boiled off first and then the positive results commence. Low moisture content is critical in gasification boilers. Below 20% or the results are less than optimal and closer to mediocre. Only in the dead of winter do I need to burn everyday. During the fall and spring the time between recharging storage can last days.
Since you are planning in advance, there are two things at the beginning. Heat loss results and preparing your firewood in advance. It sounds like you are getting log loads as we have available in the northern lower, 10 and 20 cord loads for $850/$1700. I am near Cadillac. Out here sits 3 years worth all the time. Once you get ahead of the 3 year curve, it is like one year at a time.
Once you get your heat loss results, you will be able to figure how much firewood you will need to process for one years heating.