1. You need the eMac's original disks or a newer retail package that isn't any newer than 10.5.6.
2. You need to include the disks with it. If needed, phone AppleCare and order replacements.
The installer on them will refuse to run on an eMac. If you use them to erase the eMac's internal drive, you'll be left with no way of installing an OS on it.