Insulation options... help?

So I'm finishing an attached (to my 120 yo farmhouse) carriage house/barn to be used for classes of all sorts.  There is no insulation currently, so I'm looking for good natural options.  I'm in central Massachusetts, Zone 5b/6a.  

I already have forced hot air heating that will be extended into this space.  Later, I'll maybe consider an RMH.  At the moment, the best spot for that is occupied by a behemoth electrical panel.  

I'm also getting ready to renovate a Fisher Price house for a loved one, so if I can do the two projects in one go, that would be great.  As always, price is an issue, as is time in this instance.  Looking for the cheapest, easiest, solution with the least environmental impact.  I'm likely to be doing this solo (51yo woman in reasonable physical shape).

Thanks heaps,

Sounds like a challenging project, Nissa. Please provide some additional information about the building and your natural resources such as:
Building dimensions
Floor material
Wall materials and height
Sun exposure and directions
Window and door material descriptions
Roof system
Natural materials in your landscape or area such as straw, cedar, pine needles, horse hair, sod, clay, stones, and possibly some scrap lumber.