Ran diagnostics myself which showed a failed hard drive, then used Apple Tech Support, then a Genius at a retail store. Everyone agreed, the hard drive died! The iMac was over 6 years old so the retail store could not work on it and I didn't want to wait, or pay for, an authorized repair center to do the work. I've built PC's before, and was curious to crack the iMac open. After reviewing the video's and guides on ifixit.com I became confident that I could do this.
The order process on ifixit.com was easy. I ordered a replacement drive, additional memory, and some tools that were recomended that really made the job much easier and I received the package exactly when it was supposed to arrive. I followed the guide on my iPad, while I worked, and it was very straight forward with no surprises.
Have no fear. With a company, product, and service like ifixit.com, they really have already done the hard work for you.