Ditto what DisneyMagic73 said. We went second week of September last year and did the Fantasmic dinner package, and felt we wasted money and time. We could have had better seats and gotten there much later if we had just walked in. We spent a full hour sitting on those hard, backless concrete benches waiting for the show. If we go again this September, we'll plan to arrive 20-30 minutes before the start of the show.
Doubt if you'll need a sweater. September can still be pretty hot, but watch the 10-day forecasts before you go and you'll have a better idea of what to expect.
Have a great time!!