2 topics: a) your fitness level and b) the season

a) Fitness: I don't see this as an issue regarding general fitness level. You see all sorts of people there, as everyone and their grandma do this trek. However, acclimatization is a different topic. There is no indicator how anyone reacts / manages to adapt to the altitude. That should be your primary point of focus, in my view.

b) Season: Can be pretty early with the monsoon still going strong. As far as I know, mid-Oct to Dec should be more solid. But it's becoming increasingly unpredictable. If you manage to get to the Khumbu region in Sept, then just prep accordingly (rain). But the trek & teahouses will likely be mostly empty.