if you have questions about weather, check out weather.com. they have a page for each city with historical averages for each month. Heres the link for Paris:
Should be in the mid-50s in October.. ..paris is just like any place though.. ..weather is unpredictable! you should check out the weather page a couple weeks before you go for the 10 day - 1 month forecast to check out what to pack!
September is one of the best months to travel in the northern hemisphere as the weather is typically fairly stable and all the schools have gone back. I have been hiking in the Pyrenees on two occassions in perfect weather at this time.