Well, the original G5s shipped with Panther, so there is a chance that the drivers for your iMac are in the standard Panther disc. If this is the case, follow gavinstubbs09's post, connect the FireWire cable and put the mini into TDM. This way you can use the mini's internal ODD to install the OS.
Now, you do have all 3 Panther CDs correct?
If that doesn't work, you would have to buy a later retail copy of Panther (10.3.5 was the last sold for Retail, IIRC). I would recommend installing 10.4 or 10.5. Since it is a G5 it does 64-bit and 10.5 Leopard runs great on it. Apple no longer allows 10.3 to connect to the Software Update server, so the updates have to be installed manually.
Now, you do have all 3 Panther CDs correct?
If that doesn't work, you would have to buy a later retail copy of Panther (10.3.5 was the last sold for Retail, IIRC). I would recommend installing 10.4 or 10.5. Since it is a G5 it does 64-bit and 10.5 Leopard runs great on it. Apple no longer allows 10.3 to connect to the Software Update server, so the updates have to be installed manually.