I bought a Dual 1.25 G4 (FW800) which requires a special AHT disk v2.0.1. I was able to get it by going to a local Apple store and getting the tech to look up my serial on his in-house computer. I don't know if all techs have access to these disk images, but he did. But yes, it is not easy to get. Regarding the AHT vs. TechToolPro, I don't know about the latter (only the freebie TechTool Deluxe), but I found the AHT to be a very good tool for diagnostics of the most important components. TechTool's (Deluxe or Pro) HD surface scan is a good test of the HD. The ultimate ram test is ther freebie Rember, but be forewarned, testing just 1 gig of ram takes about 2 hours on my G4.
If you have any Power Mac G4 from the Gigabit Ethernet through PowerMac G4 Mirrored Drive Doors(but excluding the FW800 models), you can download the AHT from here:
http://www.info.apple.com/support/aht.html
As far as OS install disks, for my dual 1.25 G4, I chose to install from an old Tiger 10.4.0 disk and then run the free Apple combo updater to 10.4.11. And, as I said, I eventually got the AHT disk for this G4. If you have an 867 mhz G4 or above, you can always run Leopard.