The word insulated is incomplete without a build-up. Ask what is fitted in the walls, floor and roof, how thick it is, and how the glazing, doors and ventilation are handled.
Find the weak surfaces
Wall insulation alone does not turn a light garden building into a comfortable winter room. Heat also moves through the roof, floor, glazing and air gaps. Moisture needs a safe route out.
Match the building to the use
Occasional shoulder-season use is a different brief from five working days every week. For regular winter occupation, compare the construction with a purpose-built garden room rather than assuming later upgrades will close every gap.
Read summerhouse vs garden room before paying for an upgrade whose specification is unclear.