Originally Posted by AKTrainum
some work on the walls/windows as well as fill holes and paint the metal floor.
You can strip rust off the floor, but you probably need to wait for a few days of warm weather for the rust converter and paint. Filling holes depends on the technique. Caulk won't set up properly if it's too cold (check the label), but welding is probably fine. Riveting could work also.
Then comes electric/plumbing before insulation and building the interior.
No issues with electrical.
You are probably fine to install plumbing also unless you're doing something really interesting. Testing plumbing just requires that you pick a day above freezing (or use nontoxic antifreeze).
Foam insulation requires warm temperatures. XPS/EPS boards can be cut in any weather, though glue could cause problems if you want to use it.
Building the interior is fine too for most techniques. There's no issues with carpentry, mounting appliances, etc.