Definitely go pre-made if you're looking for guaranteed quality. As awesome as friends are for stepping up to help, especially first-time attempts, generally don't turn out like you envision, it's only natural. Most things in making cosplay is trial and error
Buying a corset will also ensure that the fabric, bones, etc. are all designed to be worn, which means can hold up to a weekend of convention activities. Reputable dealers will use legitimate materials that will be worth the extra money in the long run. Looking foreword to seeing how this costume turns out, working on the same thing myself actually, for Otakon 2013.