A snapshot of the finished (but still empty) room:
There is a bit discoloration visible on the heater below the window: Apparently the radiator paint is not heat resistant enough to deal with a gas heater 😞As soon as the room was ready, I put in the IKEA Metod kitchen cabinets that act as my sub-baseboard and convenient storage area.
Note that I have fitted the small cabinet to the left of the large roof joist with wheels so that later it can be moved easily if needed, for example, to get access to the space beyond the cabinets if a train falls of or something.The baseboard itself will be the next step.