A cheap (and reusable) way to do it is get a rear disc wheel and use a bolt-on cog to bolt to the disc mount. You can probably pick up a wheel for less than the Fixxer ($80?), and it requires a lot less hub surgery. You may even have a spare laying around.

As far as gearing, I'd go up if anything. I ride offroad with a 32-20 because I am a big sally and I like to spin. On a fixed gear, I'd be more worried about spinning out of control on a descent than having to hike-a-bike. Maybe a 32-16? I rolled with that gear for a while, and I had to get rolling pretty quick before I spun out.