As I am leaving on the 23.12, you should be available on that day before 15h to be handed over the key in Berlin-Mitte (as I work there).
My studio is 15 minutes away with public transport from Friedrichstraße. Lots of cosy and cheap bars and places to eat nearby! My studio is in the back of the building and it's very quiet. It's ideal for people visiting the city due to good public transport connections.
The kitchen is a bit old, but everything works properly.