I think September would be really nice for a 5k, not too hot and not too cold. I am running one this Saturday and it's supposed to be 103, we've had a hot summer, like most folks in the country.
I would definitely get out on the road a few times and practice before your race. The road is much harder to run on than the treadmill. The more you train, the faster you will get. My last 5k I ran it in 33:32. I am hoping to beat this time by a minute this time around.
Also, most trainers recommend that you try running beyond the distance of the race you are training for. That way come race day, it will seem easier. I made it to 5 miles the other day. This will increase your endurance.
Here are some great websites I visit:
http://www.active.com/running/
http://www.coolrunning.com/engine/2/2_3/181.shtml
http://www.runnersworld.com/
Good luck!