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Hi Joe, You need to install 10.6.8 first...

You can upgrade to OS X El Capitan from OS X Snow Leopard or later on any of the following Mac models. Your Mac also needs at least 2GB of memory and 8.8GB of available storage space.

MacBook introduced in 2009 or later, plus MacBook (13-inch, Aluminum, Late 2008)

MacBook Air introduced in late 2008 or later

MacBook Pro introduced in mid 2007 or later

Mac mini introduced in early 2009 or later

iMac introduced in mid 2007 or later

Mac Pro introduced in early 2008 or later

Xserve models introduced in early 2009

Ah, this is always a “fun” mess.

Old (used) Macs can be flaky, and old disks can (and do) fail, memory sticks can be pillaged, parts are scarce, and old OS X software can variously be difficult to get. And OS X this far back will have increasing problems just making secure network connections, and with getting copies of apps that’ll still work.

Swap the hard disk. I’d probably scrounge and pre-emptively swap for a newer hard disk here (3.0 Gbps SATA), though I usually go for SSD. To HDD and not SSD here because of the age and likely longevity of these old iMacs, and the unknown state of the hardware. Not spending much, here... Not until these are known to be working,

I’d suggest running the hardware test, but you won’t have the installation disks.

Use a second (third) Mac to build a bootable installer with El Capitan. (iMac from 2007 and 2008 should be able to boot that, as was mentioned.) Transfer that installer to these old Macs, wipe, and try booting it.