We stayed at Serdar's place for a month, and we can assure it has everything you'd need for a long stay.
The space was very comfortable and cozy, perfect for two people, with the bedroom being separate from the joined living room and kitchen.
The latter comes with all sorts of pots, cuttlery, spices, oil, pasta and anything you'd need to start cooking right away, as well as cleaning supplies.
All the electrical appliances work perfectly (including an amazing AC that really saved us from the summer heat).
The internet speed was more than decent, we needed to work during our stay and we had no problems with it whatsoever.
The apartment is conveniently located, being next to the always busy Istiklal street where you'll find all the stores and restaurants you'd need, but surprisingly none of the noise coming from it can be heard in the apartment, it was very quiet.
If it's your first time in Istanbul, especially if you arrive at night, getting to the apartment might be a bit intimidating as you'll have to go through a couple of narrow streets, but there is nothing there to be scared of, only some shops, a couple of picturesque cafes and lots of friendly cats.
Serdar was a wonderful host who replied very quickly to any question or concern we had. He really helped us feel safe and well taken care of during our stay, we really appreciate it.
We highly recommend this place to anyone who wants to spend some time in Istanbul. Whenever we come back, we'll make sure to book it again if it is available :)