Q: What is the latest OS 10 that you can put on an iMac Power PC G5?

Mac OS X 10.5.8.

(135761)

Which exact Brother Printer is it?

I can't find what printer languages it uses, but I bet older one would work, or possibly the Generic postscript driver.

As far as Scanning, perhaps the Vuescan App would work, it supports many MFC models...

http://www.hamrick.com/vuescan/brother.html

And older version of CUPS printing may support the printer.  See my FAQ* on printer drivers:

http://www.macmaps.com/printersx.htm

Obviously any Intel Mac version of CUPS is unsupported by your computer.

* Links to my pages may give me compensation

As far as getting the two Macs to converse, Thunderbolt supports a Thunderbolt to Firewire adapter.   You can even get a Firewire 800 to 400 cable to hook up to your Thunderbolt adapter.

How to use and troubleshoot FireWire target disk mode - Apple Support

USB 3 supports a USB 3 to ethernet adapter.   Also you can temporarily setup an 802.11b or 802.11g network on your iMac, if you can get the original Airport Extreme card for it (if it isn't already installed).  Do not install the original airport card.   To see the difference, see these articles:

https://web.archive.org/web/20140228181928/http://support.apple.com/kb/ht1621

https://web.archive.org/web/20140228140320/http://support.apple.com/kb/HT3024

Here's an article on setting up a network between the Macs:

Creating a small Ethernet network - Apple Support

Lastly, as long as you make some printer driver connection, you can print with a save as PDF function in the print dialog.  This will allow you to put the PDF on a USB Flash drive and move it over to the other Mac.  Note, it is important the Flash drive either is store formatted, or formatted for the PowerPC Mac as indicated here:

https://discussions.apple.com/docs/DOC-3003