Step one so far - took a look inside, cleaned out an incredible amount of dust (which fortunately doesn't appear to be cigarette or animal hair based), pulled the 256mb DIMM and bunged in the 512mb I pulled from the mac mini G4 a few weeks ago. Tried again, it's running well at 1mb RAM but unfortunately the drive still isn't working - it opens and closes but doesn't read.
Step 2 will be pull a drive from my DVD copier and see if it works with that - there's enough slack in the cable to try it without taking the drive assembly apart to get at the optical drive, which is a PITA. Assuming it works, I'll then look at getting something suitable, finding or fixing the keyboard, etc. etc. No rush with this, I want to take it slow and careful.
later - CEX appear to sell drives for £4, I'll just go and buy one tomorrow rather than messing around with the DVD copier.
later still - looks like the keyboard is working, as tested on my Macbook Pro; for some reason the iMac isn't recognising that one corner of the keyboard. May just be a configuration issue; if not, it's fine with a slightly older Mac keyboard with a slightly different layout that was originally from a big Power PC, so I'll use that one instead and keep the more recent one for the Mac Mini G4.
End goal should be some profit, but I'd imagine there'll be a few pitfalls along the way.