Turkey is a fabulous place to visit. A real culture shock, yet safe. The food is good, the water clean, the accommodations can be cheap, altho you get what you pay for, and there is no personal crime. My sister lived there for over 14 years and never had a problem. I have visited twice and found it wonderful. Istanbul is fairly easy altho language may be a problem. As my sister spoke Turkish, I had no problems, but the first time I went (1993) I heard very little English. The second time, about 5 years ago, I heard more English. German gets you a long way, tho. Of course, the rug merchants at the Grand Bizarre will know English. However, buy your rugs in the US. They're cheaper there. If you can get to the source of the rugs, then they can be very cheap, but this is far from Istanbul.
Sophia allows flash photography, but no tripods ('camera with legs'). The only way to use is tripod is to write in advance and get a 4 hour permit. And that permit is for a specific 4 hours or a specific day and you must write some 3 months in advance...at least that how it was when I was there. The Blue Mosque is the same altho I don't know about the permits there. Other mosques allow tripods but no flash.
Access is easy and there is no worry about insurance and that crap. I found that if I wanted to take a pic of something odd, I could usually just ask and get permission. As a 'professional photographer' your only threat is to their postcard photographers and then only at places like Blue Mosque, etc. That seems to be how they see it.
If I were to go again, I would take only a 6x7 system. I took 4x5 one time and it was a bunch of hassles. I feel as though I'd take more pix with a smaller camera. For the mosque or Sophia, I'd take a bean bag. Setting up a tripod some place would be okay and others impossible with the crowds of people. Stay as long as you can.