What software do you want to use?
What MIDI devices?
Are the devices and software still available and affordable?
Are plug ins still available?
Will you be happy with a dead end purchase and with devices and software that will likely NOT move to Intel machines should you desire one?
Answer these with regard to PPC Mac and OS X 10.5.8 (no later available) compatibility and you will begin to see whether or not it is a viable path......