Buy.
The price of a conversion adds up a lot faster than you might think.
Even your aim is to learn a whole lot about building a fixed gear bicycle without buying a frame, just buy a complete bike, tear it apart, and rebuild it. (This was my goal when I did a conversion, and though I learned a lot, it was not always an entirely pleasant experience.)

A friend of mine just got the Dawes SST, and for the price (~$320), it's a fine bike --- especially if you're new to the game. If you end up loving fixed gear riding, put your inexpensive complete bike on craigslist, recoup as much as you can, and build whatever you want.