I personally recommend buying, then upgrading parts once you ride it and learn how the components work, and how you want to ride your bike.

Building a ss/fg when you don't know anything can be very very costly, and when you don't know much you'll end up buying cheap components. So you might as well get a cheap bikesdirect bike, or shop around for an entry level ssfg, it will be cheaper.

Conversions are also an option. However, with the spike in popularity of ssfg, many frames worth converting are already snatched up on craigslist, or are overpriced due to the popularity.