Ah yes, I think I've muddied the waters of what's already been a long-running saga. I'll collect it all together and rewind.
The first photo I've attached is of my (partially disassembled) original G4, which I think dates before the QS models to 2001 (a Digital Audio model, maybe?). I have all 5 software restore discs for it, plus OS 9.2.1 and OS 10.0.4
The second photo is the refurbished MDD, which I bought without understanding the complexities of the range of G4 models. I got no discs with this at all. It also came without an HD, but with everything else inside, including an optical drive.
Just to run down what I've tried so far; I took the HD from my original G4 and put it inside the MDD (I have all the data from that drive backed up in its entirety on an old outboard external HD), with the intention of wiping it, installing 9.2.1 on there and setting back up again. I found I was able to boot into OS9 using the software install disc, but could not get the installer to run (obviously). I couldn't even boot off the OSX disc, getting a 'the installer has unexpectedly quit (error 1)' message. I've also tried downloading an OS9.2.2 DMG and installing from that, but the MDD hasn't recognised either the boot disc I made, or the memory stick with it on.
The ebay link was to a machine which looked a lot like my original G4 and which I hoped would solve all my problems, but as far as OS 9.2.1 goes, it seems not. Logic Audio 4.8.1 (which is the application I need access to) may run under 9.2.2, but I have no discs for that, and the DMG download I haven't managed to make work with the MDD, as I mentioned.
Hope that makes things a bit clearer!