El Capitan & A1225 compatibility

First iMac, be gentle please!

30 year PC user so I learn fast, have plenty of iPad & iPhone experience

Got a deal on this A1225, it's this model

http://www.everymac.com/systems/apple/imac/specs/imac-core-2-duo-2.4-24-inch-alu minum-specs.html

Got it home, fired up, seemed fine. Said had an iOS update (was already running El Capitan) and afterwards it's now running 10.11. So I'm running Safari and checking things out, it locks up. Everything is dead except the mouse, which doesn't do anything. If it matters I had only logged into my Cozi Calendar and that seemed to be the issue as I am here on safari now, no problems.

Hold power button, reboot, didn't see that it would reload apps so when it came back up the + key appeared to be stuck and it was filling that in everywhere, locked up right away, same thing. I caught it next time and managed to stop the + sign issue but locked up again, grab phone and google it, cmd+alt+shift+esc did nothing, neither did any other key combos recommended.

Locked up again when I tried to open something else after a reboot, but I don't think I had killed the Cozi tab...

Read that it might be an iOS compatibility issue but the only thread here I could find was pretty old. So what can be done, anything? It's from a PC store so there is no version to roll back to, I read that Snow Leopard was the latest compatible version for this iMac

Any help/insight would be greatly appreciated! And like I said, be gentle! We're all iPhone and iPad but W7 PCs here. First go at the iMac, couldn't pass up $200. However I think I need a better mouse, this one button thing with no scroll wheel is driving me nuts

Bud