I know there are a couple scattered threads on here regarding towing, only because I searched quite a bit for myself. But I'm just going to list a bunch of stuff so it's easily searchable in one thread.
I have a 2009 2.5i and I used a Curt 12284 Class 2 hitch. It is the same as the Draw-Tite 36334, U-haul 36330, and the Hidden Hitch 90138. I paired it with a Curt 40003 2" ball.
I'm on spec.B Bilsteins and Rallitek springs, with an AVO 20mm RSB. Also running EBC Red stuff pads front and back. Everything else is stock.
Installation is very simple. Drop both mufflers and take out the rubber plugs that are covering the bolts in the unibody. Hitch bolts up easily with the included hardware.
Unfortunately, there is no plug and play wiring option for the 2.5i or 2.5GT sedan, but wagons, Legacy or Outback, have a harness hidden away in the back that you can buy a plug and play kit for.
I used the Curt 59146 Powered Converter. Again, all the hardware and wiring is included. I just ran the positive line along the hood release cable and pushed it through the existing grommet. Then it was as simple as stuffing the cable under the trim on the drivers side and to the trunk. I left my converter box behind the little cover that gives you access to the taillight.
I only towed around 1000 lbs for 500 miles and it wasn't too bad at all. Handling and braking wise, I didn't really notice the trailer was there, and even with only the N/A 2.5L, it didn't suffer TOO badly in acceleration and power. I could easily still cruise at 70 mph on the freeway.
I know people are going to say absolutely do not tow with our cars, but I had to make do with what I have. People in Europe tow their caravans with their tiny cars and they're ok for the most part, just make sure everything maintenance wise is taken care of.