The problem is you need to do what the following article states. Since your Apple ID didn't install the OS, you can't download it. If you never downloaded Yosemite using your Apple ID, it is no longer available. Try booting into Internet Recovery (command - option/alt - R on a restart), erase the drive, and then reinstall the OS. It will install the OS the computer shipped with. You can then upgrade from there.
Selling old Mac (4) Apple support
Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.
Application Compatibility
Applications Compatibility (2)
El Capitan 10.11 Compatibility information
Also check to make sure there is a compatible driver for your printer.
Do a backup before installing, preferable 2 backups on 2 different drives.