I live in Trondheim so Åre is my local trails (almost).
There are plenty of good quality holiday apartments to be had, look on Skistar.com for prices but there won’t be any problems with availability.
Lift passes are about £30 a day.
Weather should be fine (but then you might find it pisses down with rain the whole time you’re there). I’ve never found myself rained off the hill anyway.
The downside with September is that most of the lifts have closed. World cup lift is open during the week and the gondola is open Friday to Sunday. The trails from world cup lift are good but they’re pretty cut up after all the people on rental bikes spent the summer braking in the corners. I could do one day just doing those trails but would get a bit bored after that.
When the gondola is running it’s great. The trails from the top of the mountain are fantastic. Some of the best trails are over to the left side of the resort but that then means a bit of a ride to get back to the open lifts.
June/July/August are the best times to go but September is still alright.
Let me know if there’s anything else you’d like to know.