I strongly recommand to stay at Barbaros place, we really enjoyed our 5 days in Istanbul!
Barbaros is a really kind and helpful and reliable person, you can trust him. He speeks perfectly german, and also english.
The room is very comfortable, clean and nice, the bed and sofa are comfortable. It looks exactly the same as the pictures.
It is well located in Kumkapi (Sultanahmet), you can have a nice walk to the famous places (Ayasofya, Blue mosquee, Topkapi palace, Grand bazar...) or simply take the tramway if you're tired, the closest station is around 10 minutes walk. It is also easy to go with the tramway to Galata bridge and Taksim place.
Thanks again Barbaros!