I am a PC tech unwillingly thrust into a tech position at a ccompany who has 100's of Macs. They have every variant of Mac from an old Power PC 800 to the latest G5. Some of the apps they use run in classic mode only so they have to use OSX 10.3 Panther, is it true that the newer G5's won't support Panther at all?
I have very little experience with OS9 programs, so don't take this as the final word BUT...I think the deal with the G5s is that they cannot boot into OS9 directly. I'm nearly positive that the classic emulation mode still works in them, however, so they should be just fine for what you want to do even if you upgrade all the way to Tiger.
That's what I thought, anyway, hopefully someone else with actual classic-mode experience can give you the final word on that.
Classic should still work fine in OS X v10.4.2 (Tiger). However, make an effort to find them new, replacement applications as soon as possible. They are living in the past and tech-support on very old systems/software is a nightmare.
Just about every new Mac shipped requires the current OS at the time of its release. So no, you won't be able to run an older version of OS X on a new G5.