How many people will you be in total? I've done some editing projects for a tour guide who seemed professional and knowledgeable whose English was fine. At any rate, his prices are for 4+ people.
His name is Ergin and here is his website:
http://www.yourguideinistanbul.com/toursfees.htm
If you do contact him, please tell him Collette referred you.
Good luck finding a tour!