Karen's place is the best around Berlin to get where you need and do what you need to do. No wifi, but not really much of an issue. Lot's to do, 7 minute walk from a club that's open until 6AM, it's rad. Highly recommended. Karen is very responsive and helpful and will help you make it a great stay.
Make sure to ask about the door as it is particular, but it's a small issue that anyone can work around.
100% recommend, stay here if you're in Berlin!