I'd stay far away from anything pre-macbook in terms of laptops. I've been personally burned by HORRIBLE QC in Apples Ibooks, specifically the G3 ones but also certainly the G4 ones as well. An old Powerbook might be nice, but my understanding is that stuff isn't being written in universal binary any longer.
Grab a 1st gen macbook, should be pretty cheap. If they're not dead-set on a latop, I'd wait for the Mac Mini refresh, it's your cheapest path.