I can come up with WAYS to use this thing - yes, writer's retreat is a perfect example of what I'd like... or, short-term rental or something similar. It's how to get it to a point where someone (even me) would want to spend time there.
answers to questions:
Electricity: there's the beginnings of wiring, with some outlets and some totally inappropriate ceiling lights. We're off grid ourselves, and this structure is too far - 200 yards? - away from our house AND our house has as a just-big-enough solar array for us. And, there's no real way to even put solar panels on this thing, or near it, given the hill and the trees. That wiring was designed to run with a gas gen. (lovely, right?)
Plumbing: zero. no in, no out. I'd do composting toilet anyway.
Propane: if I get this thing up and running, yes, a propane tank would be part of the plan, for water heat, stove, fridge.
Outside sheathing: yes, you're right, that's not OSB, but that plywood designed for exterior. Still... not ideal for this permie/naturalist guy.
Insulation: yes, I can buy something, anything, but I'm hoping to get ideas on ways to use salvaged/scrounged or natural stuff. I don't need much R-value, as it is CA, but in winter, this thing sits in the shade all day. cold and damp doesn't make for a happy writer.
Loft: no, there's no stairs. yes, I think it was set up as a sleeping loft or storage area. Still, there's not even that much storage.
Thermal Mass: I *might* do some sort of cob-surround for a cast-iron wood stove, but given the small space, I don't really need to. (I think the main room [2 car garage] is 20x24 = 480 sq ft) so if it was insulated enough, the stand-alone wood stove will do fine.