I would say a single processor G4 Quicksilver would work. Preferably a 800 or faster (733's came with no cache and are slow).

Super cheap bordering on free.

Used AGP cards which are inexpensive and easy to upgrade if needed.

Is a screaming OS 9 machine.

Not too noisy and should not overheat.

Can dual boot to OSX 10.4.11 (which can use a cheap USB2 card while OS 9 cannot do anything but USB 1.x).

Only drawbacks would be to make sure the PSU is working. Even if the unit is missing a HD/CDROM and RAM those are super cheap to source (standard IDE and PC133 RAM).