I am an Australian citizen and my fiancé is an American citizen. I have accumulated student/credit loans. Will this affect my application for a K1 Visa?

No, you should be fine for the K-1, your fiancée will need to execute an affidavit of support when your AOS is filed.

Agreed. The income and tax returns of the Petitioner/U.S. citizen is all that considered at the interview. Your credit history will not be examined.