Can't reinstall OS X (iMac 2007 - Yosemite)

So, I bought an 2007 iMac second hand of eBay this weekend and can't get up an running on it.

When I switch the Mac on, I am presented with the OS X Utilities page giving me the option to 'Restore From Time Machine Backup', 'Reinstall OS X', 'Get Help Online' and 'Disk Utility'.

My first natural choice was to 'Reinstall OS X' which, if I follow the steps, tries to install OS X Yosemite on the device but when I sign in to my Apple Account I get the error 'Item temporarily Unavailable'. After reading around, this seems a common issue as I've never owned any OS X versions so don't have the availability in my account.

The trouble is i'm at a total loss as to what to do next!

Things I've tried:

• Boot To Internet Recovery mode - I've tried every suggested key combination but can't access this - not even sure if this was available on 2007 iMacs?
• Follow through the steps for 'Get Help Online' - there's some suggestions on the web page, like trying to 'Customise' the install back to Mavericks but I don't see these options.
• Wipe / restore the disk with 'Disk Utility' - I tried to restore the main drive but I think this is just copying across the Yosemite files again which isn't helping, doing a complete wipe of the disk didn't help either. I even tried to restore from an image of Mavericks over the net (from the Apple site) but I kept getting problems with scanning the destination saying it was 'Busy'.

To sum up, i'm totally confused how to move forward with this. I thought it would be a simple job just getting the Mac setup under my account and i'm totally amazed at what a painful experience it's been already.

Any help gratefully received. Thanks.