I've climbed in mid-Sept (about 8 years ago). The evening buses still operated from Tokyo to the 5th station (not sure about nowadays though). Most places at 5th station were open, including toilets. Most huts on the trail were closed, but a couple were still open. Many other people were also climbing, but no conga-lines (I went mid-week). I don't know if they will still collect the new entry fee in September (I think its 1000yen, but it got created in the years after I last went). It was colder than summer, but not enough for snow.

One consideration is the longer nights. In summer, the sun rises up to an hour earlier than in mid-Sept, which can help if moving real slow and needing more time to reach the top for sunrise, but if moving at a normal pace (again, for me there were no crowds to wait behind), it meant waiting and shivering for an hour+ at top. I even waited at the 5th station till a pretty late time (about 10pm) before embarking, but I still got to the summit well before sunrise and had to wait in the chilly wind. So bring some heat patches (hokkairo) if you can find them (but they're not usually in stores yet in Sept).

Unless wanting some kind of peace of mind (for possible injury, altitude sickness, etc) then a guide is not necessary.