Take the Gouter route. It's the regular and most used route up the mountain. It's a bit of a slog though! We did it around the 20th-21st of September last year. The route and hut wasn't crowded and the weather was fantastic. In sept, it's the off season, so you get less people on the mountain, and a more reasonable rate in the hotels in Chamonix. The Priory is a nice hotel, and reasonable in Sept.
Re Guides, there's an Irish guide named Robbie Fenlon that lives in Chamonix and he gives a six day course covering all the basics of ropework, ice axe and crampon work etc, as well as doing a couple of other 4000ers as a means of acclimitisation. The course finishes with a two day ascent of Mont Blanc. Check out his website at www.wilderplaces.com
. For somebody with no experience of high altitude mountaineering before last Sept, I found him excellent.