I come from a place whose Summer light is similar to Istanbul's, just much stronger. I shoot 400 ISO film all the time in Summer and I have never, ever used f/16 or f/22 in my exposures. I also have never owned an ND filter. Probably a matter of taste in terms of target exposure. I like to pull.
Curious you'd use the word 'versatility', I love 100 ISO film, but it's 100 ISO film that doesn't give me as much versatility as 400 ISO film in strong light/handheld Summer photography.
Sorry, did not want to make it sound as if I was making a rule out of it. A lot of people feel the way you do. It's a matter of habit, I guess. And what pleases you most in terms of look, as you mention.
When I travel south, or even here (Montreal) in the summer, I now mostly carry FP4+. I find it very versatile as it can be shot at 65, 100, 125 and 200, all with excellent quality. I really like it at 200, much more so than I like Tri-X at 200, which is how many people rate it.
That said, a lot of it also depends on the camera. When traveling, I only take the 35mm, so a low ISO film works well. If I'm to shoot with the heavier Pentax 67, then yes, I also go with a 400 ISO film, HP5+ usually.
As mentioned above it is meant to be a documentary project along a storyline, not "street". B/w because i really like Ara Güler's book about Istanbul.
This means you have to think about this as a mid- to long-term project. One doesn't document a city in a couple of days. We're talking months, if not years. Ara Güler is a good example — he spent his whole life there —, another being W. Eugene Smith, who was hired to do a documentary on Pittsburgh in a few weeks, and ended up spending three years there.
In order to take out gear as a source of worry, main question for you would be is this a place you can go back to often or are you there just once. Makes a difference.
My experience is that when traveling one always, ALWAYS, run into a situation in which one says "I really wish I had my [
insert name and focal distance of lens here] with me for this shot!". Always. Not to mention finding the perfect spot but the day being cloudy, or the sun shining in the wrong direction — the ol' "This scene will be absolutely stunning at 7 pm but my plane is leaving at 5" trick.
This stops becoming a problem if you know you can come back another day, another time, in a week, a month or two, the next year. If so, traveling with just one lens stops becoming an issue. Shoot what fits with the lens you have, always carry a small notebook with you, take note of the place, time and the lens that you think would work for the shot.
Advice would be different if you only had a few days in the place. For travel, when I know I won't have much time and might not have the chance to come back, I take the 35mm camera with a 35mm (or 28mm), a 50mm and an 85mm lens. It never covers everything, but it covers most of everything.
Same with film. As I mentioned above, I like shooting slow films. Albiero, and others, shoot fast films in the same situation. There's not right or wrong. There's just experience, taste and practice. Take your 400 ISO film, see if it works for you. If it does, great. If it doesn't, next time you're going there, take another, slower, film. Or take both if experience tells you you need both because there are too many different lighting situations.
Side note : another photographer who brilliantly captured Istanbul, this time in color, is Alex Webb. His book on the subject is marvelous.