Wear whatever shoes you think will be comfortable to walk in for long distances and for the love of all that is holy, if you're going to buy new ones, do NOT purchase a pair less than six weeks before you go!! You will need time to break them in. The max I walked was probably 10-15 miles a day, averaging more like around 8-10. But I also have a job that requires me to be constantly moving, so I was already somewhat used to that and had a fantastic pair of shoes. And I *did* take public transportation fairly often, lol.
Stay in whatever dorm rooms you're most comfortble in.
If you're not sure about going co-ed, book yourself into a female dorm. If you feel up to venturing to the wild side, you can switch over.
Usually I don't really care where I end up; I've stayed in all female rooms, half & half mixed, mostly guys, and one extremely interesting time when I was the only girl in an 8-bed dorm with 7 dudes from Sweden who were in London to catch the ABBA musical.
There was never any point where I felt even remotely unsafe, but then again I'm also not always the most cautious or observant person. I've wandered, somewhat oblivious, into a good number of crackhead or otherwise ill-advised neighborhoods by myself.
With no ill effects.
(Actually the worst experiences I had with being bothered while wandering around were usually in the touristy areas. But that's western Europe, I'm betting New Zealand is easier to deal with!)
And don't bother with an iron. A hostel *may* have one if you're really desperate, but just try to bring clothes that don't require one. Or put your stuff in a dryer for a few minutes if you're desperate and nothing else is available.