if I were to plan a once in a lifetime trip to DL in Sept and wanted to be sure to experience the Haunted Mansion, what dates should I choose? How long is HM down for the Halloween overlay to be done?
You will only experience the Holiday Haunted Mansion in September. It usually reopens as the Holiday Mansion the second weekend in September. The regular HM usually closes after the 3rd weekend in August.
Yes, that would be a good bet to see the Haunted Mansion Holiday version which is a complete redress of the Haunted Mansion done up Jack Skellington style from the movie Nightmare Before Christmas. If you want to see the traditional Haunted Mansion, then late August - mid-January is not the time to go. You would need to visit sometime between mid to late January through mid-August.