The computer and its operating system, the DMXadapter and its drivers, and the lighting software all have to be compatible. That's easy to do with current products, but fairly difficult to achieve by piecing older stuff together.
Consider whether you want to risk shows on elderly computers. The iMac G4 is at least 14 years old. Even if you find one that runs, every boot up could be its last. Hard drives more than five years old are extremely unreliable.
Look for a computer that is just a couple of years old, running Windows 7 or 10. Current software and DMX adapters should work well with it, and you'll have something you can trust a show to.
Its definitely possible to find something that works on an old PC, like Hog 2 PC, but thats not for Mac.
Difficulty with using old Macs is that they are PowerPC, not Intel, so drivers had to be written more specifically for that processor.
As a Mac person, I have 2 PowerMac G4s and an iMac G4 lampshade. Beautiful industrial design.
1987, an old console with memory dies. West Systems? to the rescuce with an output card for an IBM XT. They run just a few wires to the dinners to replace the 96 pairs of 0-10v control. I learned what "multiplex" means!