Keiran's house was in the best location. Close walking proximity to all of the major sites, Bosphorus and great cafes / restaurants / street food. I would suggest staying here if you're more inclined towards exploring cool neighbourhoods over the tourist grind.
The loft itself is spacious, well equipped to stay for a few days and bed extremely comfortable. Good space to work also if need be.
Don't worry about the heat - even in early summer we slept soundly and were never too hot at night. The only issue we experienced was slow Wifi which made working from the apartment difficult though Keiran was quick to communicate and try to fix this.