If you can't find a Seagull PC-9 ARF to fly, you might consider the Seagull Spacewalker II. It's a great airframe by most any measure. Of the three you mentioned however, I'd pick the the T-34 Mentor for being the best blend of easy to assemble, fun to fly, and stable/easy to land. The Hangar 9 T-34 ARF is a nice airframe. The folks at www.airborne-models.com carry the World Models T-34 Mentor ARF, which also has a terrific reputation and is about $30 lighter on the pocket.

The Cap 580 and the Cessna 182 are a little more scale and will take more concentration to take off and land smoothly. They are both very good airframes as well, however. You can't really make a bad choice so long as you're confident that your piloting skills are up to handling them.

As far as ease of construction, all of the planes I've mentioned should be pretty straightforward assembly projects. Good luck, and good shopping!