On my first visit to IoA, it was in the dead middle of Spring Break in 2000. We did the Studios park and IoA in one day. The lines were fairly long but since the park hours were a little bit longer, we got to do pretty much everything.
We went back in Jan '05 and decided to spend 2 days at Universal, because a 2-day, 2-park ticket was only marginally higher than a 1-day, 2-park ticket, and fiancee wanted to experience both parks. Day one was spend at the Studios park where we did everything (most things multiple times) in one day, even with the park closing around 6pm. Day 2 was spent at IoA where we did the major attractions within an hour of opening and had most of the minors wrapped up an hour after that. We took in the Sinbad stunt show and did rounds on the water rides. We got bored around 4 and decided to head back to the studios park and hit Mummy a few more times and give Shrek another chance (we still didn't like it). We didn't use the Express system at IoA, but we did use it a little at the Studios, even though the standby lines weren't terrible.
I think the #1 key to avoiding lines is to get there at opening. I wouldn't spring for the express passes and I certainly wouldn't get a room there just to get them. I'm not sure the what the crowd concentration is for September, but in my experience they seem to be a bit thinner at Universal than at Disney. I think it is definitely possible to spend 1 day at Universal and see just about everything (I would recommend the Studios park when traveling with a 7 y/o).
A 1 day, 1 park ticket (for either the Studios OR IoA) can be purchased at the gate for: Adult (10+):$63.00 + tax, Child $48.00 + tax. A 2 day, 2 park ticket at the gate is $110 + tax, regardless of age. Of course you can get them cheaper in advance online.