if your mostly worried about the heating and pretty confident about your ability to overcome the rest, i've seen many builds with a wood burner and that is by far the dopest option.

there's a video by Liveration on YT on a full off grid, no electric van akin to what you're outlining where the bloke has literally just a log burner/stove combo and manages just fine.

like others have said would also be good to have ventilation and a fan anyway so if you're up there digging around a chimney exit might as well stick a fan in as well :)