Rather than rolling the whole OS back for a scanner, maybe try VueScan which is a 3rd party scanner driver for scanners that have been abandoned by their makers. It works great.
Here's a list of supported Epson scanners:
As to your install problem, it's possible that there was a firmware update that was required for 10.5 that breaks your 10.4 from running anymore.
Here's a question for Mr Mac Fixit:
I have a 17" iMac G4 that I was rung Tiger with, then I installed Leopard 10.5. I use this as a second computer. All is fine except that my Epson scanner does not work with 10.5. I've tried the latest drivers. I'd like to reinstall Tiger. I used the 10.5 disc to reformat the drive and start from square one. When I boot from my original Tiger disc it starts opening it's windows, but when I get to the point where I can choose Install or use Disk Utility to reformat from the Tiger disc, all windows freeze and I need to hard reboot from the power switch. Am I correct to assume that 10.5 won't let 10.4 to reinstall. Is this correct?