Cheap - not cheap - cheap - not cheap - hmmm . . .
Cheap minimum for OS X.x with reasonable future breathing room - AGP graphics G4 (Sawtooth) with a 32 MB Macintosh Nvidia graphics card or another AGP video card that will handle the Quartz Extreme aspect of OSX.x well. The Sawtooth comes in numerous flavours - 350-500 MHz - a good range for different budgets. Some of the video cards have DVI output, some do not - something to consider.
Lotsa RAM - RAM is cheap.
Make sure you warn him away from a PCI/G4 - a novice might mistake it for a bargain.
Noise tolerance? You may want to warn them away from the Leafblower Macs.
Moderate price range - something better.
Not cheap? Hey - sky's the limit -
Choice is good -
Epaminondas