Old but good G5 OS

Powermac G5 late 2005 -

I cannot get it to do much of anything really.

If I remember rightly, it came to me with no HDD so I bunged in a raw 250gb to install to.

I know it's meant to run 10.5 but I don't have that yet, so I thought it might load any OS X disc really. But no. It will not. I have tried a few different ones including 10.6 and even 8 but none of them will load. The Mac tries reading them but to no avail.

I have tried another known good optical drive, from a G4 still no luck.

I have the 2.5 drive from my MacBook Pro, so I thought it would load from that. I thought wrong.

I can get the firmware thingy (forgot its name) to load and I can get the screen to load using the command button, but even with 250gb blank drive, 2.5 with Yosemite installed PLUS any OS X disc in a known good optical drive, no boot devices will show up.

I have tried resetting both ram & rom or whatever they are - several times.

I have had all the ram out and tested each individual one and reseated them.

The front panel lights are doing exactly what they're meant to do.

Being from a large printing press - I wonder if the firmware has been modded to only boot from network. Does this seem likely ?

Any help appreciated. I'm pretty sure the hardware is fine, so I'd like to get this thing up & running as it's going to go to a good cause.