Will it be comfortable in January?

It should be, and that is what separates a properly built room from a summerhouse. A fully insulated, high performance build keeps heat where you want it, so a modest heater handles a cold morning. Ventilation is designed in at the same time, because an airtight room without it will end up damp. A 3m by 4m room built this way is comfortable in February and cool enough in July.