This is very much a matter of preference and taste. Henri Cartier-Bresson was inseparable from his 50. The same holds for Jean Gaumy. On the other hand photographers like Bruce Davidson and Joel Meyerowitz seem to have a preference for wider lenses like 35 and even 28. One thing is sure: Any focal length longer than 50 is not an option. An 85 will make great snapshots, but will leave no room for creative composition.

There are three factors that you may want to consider before choosing a focal length for street photography:

1. Light: Shorter lenses offer a deeper field of focus at equivalent f-stops, what makes them more usable in lower light conditions. A good share of all street photography is shot at small aperture and using hyperfocal and zone-focusing techniques, so this is critical.

2. Space: If you are shooting in narrow streets and constrained spaces you may not have much of a choice anyway (pick a short lens).

3. Composition: The shorter the lens the looser the compositions you can produce. At 28mm there are simply way too many random items present within the field of view of the lens, so a meaningful composition is more difficult to achieve. Furthermore, wide lenses exaggerate perspective and it is hard to shoot anything with them that looks straight out of the camera. On the other hand, while 50mm lenses can produce great compositions in the hands of an experienced photographer, many find 50 an unnerving focal length. That is because 50s offer a rather limited field of view and almost never allow you to fit a whole scene within a single shot. So, with a 50 you need learn to make quick choices on the spot regarding what to include within your frame and what to leave out of it. This is a great photo composition exercise as it trains your eye to identify the essence of each scene in real time. But it also means that until you get used to the exercise you will likely miss some shots, and since the field of view is tight there will be little leeway for cropping afterwards.

Now, where did your criteria go, i.e. the balance between focal length and respect of personal space?

The truth is no matter what focal length the street shooters use, they almost unanimously shoot at very close range. By close range I mean distances that would be rather uncomfortable for most of us. One of the secrets of becoming a good street shooter is overcoming our fear of causing discomfort. There are obvious cases where you do not want to shoot at a very close range, but that is not a question of comfort. It is rather a function of the type of content you want to capture. Good street photography happens, in many cases, out of most photographers' comfort zone...

What I suggest is to pick your lens based on the aesthetics you want, and then find a way to use it in the context that matters to you.

One trick is to develop the right social skills to alleviate potential discomfort or eventually handle the situations of crisis. This said, I have never experienced any crisis when shooting in the streets. In most cases bringing down the camera with a smile on your face does the job. Most people smile back at you.

Speaking of gear, make sure you use a camera setup that does not come across as too intrusive or offensive. In my experience the worst choice is a large black professional DSLR. That's paparazzi material. On the other hand, some cameras are just made to be inconspicuous. Since you are a street photography enthusiast, you have probably heard about the Leica M legend (street photographer's best friend) or maybe even about Rolleiflexes and their waist-level finder (that spares you the eye-contact with subjects). I have used both of these cameras, and while I don't find them necessary for street photography, I can attest that they fulfill their job quite well. That's probably one reason why rangefinder lookalikes (e.g. Fuji x100 series, etc.) are so popular with the new generation of street shooters.

Also, note that film cameras look very harmless in general. If you are burning film on street shots, either you really are a passionate artist, or the kind of incurable pervert that goes to such an extent to satisfy his perversions. Most people go with the former and thus perceive you as an honest and harmless artist. That makes a significant difference in the way individuals react to you.

If you are interested in street photography, Eric Kim's Street Photography Blog is a classic to start with. He has guidelines for overcoming the fear of shooting in streets, and even a comprehensive guide for purchasing equipment.