well, you have 3 types of early 2000s macs, likely 2002 era.
The MacMini might be ideal for you to transport, its small and you conect it up to any monitor, Keyboard and Mouse, the Powerbook is a complete laptop of the time, and he PowerMac is a tower system.
So The Powermac is not so portable if hat's what you want.
The Powerbook is most portable, but the Mini is small and cool if you have peripherals at two different locations.
The G4 is stll adequate for business apps, but is fading into history, if the CPU is faster than 1.0 Ghz then you can still run 10.4 well enough.
If you can find a Powerbook with a 1.2, 1.5 or 1.67 or Mini with a 1.5Ghz G4, you should do well. iBooks also came with a 1.2, 1.33 and 1.4Ghz G4 them, so those might work for you..
PowerMacs came with G4s from .350 to dual 1.4 Ghz
be sure not to pay too much for this 5 year old technology, well the mini is likely 3 or 4 at most with the G4.
The new Intel Mini is $650, so keep that in mind. sometimes Apple offeres 1.8 Ghz Core 2 Duo mini for $539 on their refurb site.
That would be your best deal and get you a basically new Mac
I find most used G4 Minies to be overpriced and sell overpriced, considering what you get. But if you find a 1.4-1.5 Ghz G4 mini for $350, I would think about it.