My brother built his one bedroom, 900 sq ft cabin using huge shipping crate pine boards probably 30 yrs ago.
Flooring, siding, and roof instead of using plywood/OSB.
Used conventional lumber to build the stick frame.
Still good as new...
There are multi-zone mini splits available.
SUPER cheap to operate...my 1 ton 21 SEER Mutsu system in a 400 sq ft un-insulated sun room works awesome.
But it does require 220v...had the heat on this morning.
Shown upper right corner...
Call around to some local auto paint supply stores.
Don't buy more than you'll need...once the can has been opened, it'll harden.
Maybe buy a couple smaller cans...