First things first:
I will buy a G4 iMac myself sometime sooner or later, but not for productivity use.
I just adore its design and the joy of old hardware.
Apart from that, I cannot stress enough how much you want a C2D Mac instead if you don't want to spend too much.
Apparently you aren't worried about being able to upgrade the OS much further, so I assume that would make a good first Mac for you.
Model? Hmmm, I'd go with an iMac with Radeon X1600.
Those babies got mass defective (the GPU that is) and if you don't get a defective model (which you could return if the seller lied to you saying it's working ok), you might even get a repaired model.
Hence the logicboard and all of its components would be newer than most other used Macs that are similarly old and cheap.
edit: too late haha
Glassed Silver:mac