so does the hard drive have to be in any specific state?
Is there an internal drive in the G5?
That drive needs to be formatted as "Mac OS Extended (Journaled"
You can do that using the install disk
what I have done was i went to a site called Mac repository and downloaded panther 10.3. it was a toast file and i read where i can just rename the extension from toast to iso. i burnt the image to a cd, put the cd in the G5 and powered it up.
AFAIK, macOS 10.3 will not fit size-wise on a CD, the install disk was on a DVD or as a special order, one could get the installer on several CD's.
So I don't know what you actually have.
But even with a complete macOS downloaded, a "burned" CD or DVD will not work on all CD or DVD drives, I think you need to get an original "pressed" DVD.
And as Nick mentioned, make sure you gtet either the black one or the specific one for the iMac G5.
I would suggest you post here before you buy
if i hold down the "C" key at boot it goes to the Apple logo and just stays there. If I just turn on the
G5 i get the folder/ question mark then it goes to another screen where it shows a clockwise arrow , an arrow pointing right that looks like it is highlighted with a frame around it then something that says Mac install media...or something close to that. then after a while I get the Apple logo. I have a PC keyboard and mouse connected. No mouse function throughout the whole process. there is a good possibility too that the G5 is broken.
With that coming up on the screen, I think the iMac G5 is fine, it just needs an operating system.
PC keyboard and mouse will work when proper macOS DVD is detected.