I have a 400MHz G4 that runs Panther fine. You need a good size hard drive and plenty of RAM, which is cheap for the G4 tower. Try to get a Sawtooth or newer machine for $300-$400. If you're going to spend more than that, the eMac will outperform it, and you won't need a separate monitor. If you get the eMac, make sure you get the tilt & swivel base.
G3 models may be cheaper, but they will be much slower. You might find a used G4 Powerbook (TiBook) that fits your budget.