Difficulty Loading Original OS X 10.4 Tiger on Aluminum iMac

Hi All

I'm have problems loading the original OS on an 20" Aluminum iMac I've been given (from reading the forums I believe that this is a mid-2007 vintage Intel iMac). The PO erased (formatted?) the HDD and gave me the original OS 10.4 DVD (2Z691-5811-A). I'd expect that I should be able to pop the DVD in and reload the OS. Upon boot all I get is the grey screen. Pressing the C key at boot gets me to the OS X UTILITIES. But what should I do from here?

I'm thinking that this iMac was previously set up with OS X 10.10 (Yosemite). As when I access 'Reinstall OS X' from the 'OS X Utilities' menu, the only option I get is "OS X Yosemite To set up the installation of OS X 10.10, click Continue." When I do that, It asks me to agree with the terms and then asks which disk to install to (iMAc HD, Mac OSX Install DVD or Recovery HD). The only option is the iMac HD as the Recovery HD is locked. I try selecting the iMac HD and I'm prompted for my Apple ID & PW. When I provide this and sign in I get "This item is temporarily unavailable". As I've tried this over several days, receiving the same message, I suspect that I'm running into a registration issue as my apple ID doesn't match with a OS X 10.10 purchase. BTW... I successfully log on to my home WiFi before I attempt this.

To summarize... I'm trying to downgrade the OS back to OS X 10.4 on an Aluminum iMac. I only plan on using the iMac to sync my family's iPhones (5s) and be a music repository.

I'm a wiz at PCs, however this is my first Apple computer, so please don't assume I know anything. 🙂