Well, let's just assume you have some cash lying around if you're upgrading after one year-ish.

I'd buy a White Industries ENO eccentric fixed gear rear hub. This will allow you to have proper chain tension in your vertical dropouts. Lace that to the rim of your choice. Purchase a cog and lockring Then, get a crankset with the proper chainline (42mm, typically) and you're ready to roll. Enjoy!