install old OS on an old iMac

I just 'inherited' a 2007-era iMac, with a wiped-clean hard drive. I would like to install some version of OSX on the iMac just to get the thing running. My main Macs are both running Sierra, which I believe is not compatible with the old machine.

I have been able to connect to the iMac in Target Disk mode, but I'm not sure what to do next. My most recent install DVD is from Leopard (and it's pretty scratched up, so I'm doubtful).I've downloaded installers from Lion and El Capitan but they won't boot on a machine running Sierra...

Any tips?