I don't know about conventions, but I believe the Epcot's International Food and Wine Festival is going on that time, as well as Mickey's Not So Scary Halloween Party. There is a link to Upcoming Events on the main DIS page you can look at for more information.
October crowds aren't bad at all, unless you go Columbus Day weekend, in which case it's crowded then for that time of year.
We always went in October until recently (my dd is too old to take of school now) and always enjoyed the thin crowds and cooler weather.
Enjoy your trip!
Nancy