For 150$ you may get any PPC (except the Kolibris) you want. But that's not the point.
os8/9 will run natively on all of the G3 and some of the G4 hardware.
You may go for a PowerMac+Monitor or the all-in-one iMac or a NoteBook. They'll be great machines for os8/9.
There are the big boxes aka PowerMac G3/G4: check everymac.com, if your model of choice will boot into os9. All of them will perform nice with os8/9. The CubeG4 is kind of special (mainly, when it comes to the price)
You'll have a lot of stuff to carry around.
The iMacG3 or even the early iMacG4 are as awkward to handle like the PowerMac-boxes/monitor/cables,
but they do come in one piece and they've got a handle to carry them around and also to stuff them away temporarily. That's a big advantage plus they do look really nice. But keep off the G3 tray-loaders! Their fans don't go to sleeping mode and you'll have a constantly humming Mac in your place until you switch off power.
Me, I'm a fan of the books ...
I do not fancy the G3-PowerBooks and I won't ever go for one of them, but considering "form follows function" they were top of crowd and haunted by a possee of copycats. I happened to ow one of those - a 2003 Acer Travelmate ... that's enough about that. I still like that Travelmate, but I do not want to go the way back to the G3-Powerbooks.
The G3-Clamshell-iBooks are unique. Screen-size/resolution is limited, but ok for os8/9. FireWire really comes in handy, especially if the optical-drive is struggling.
As
@bunnspecial said: the white G3-iBooks "snow" are great machines. Better resolution compared to the Clamshells. OSX will also run decently. But you'll have to cope with the smell.
Finally the TiBook G4: blazing screen&performance, high-tech Titanium attached to a plastic frame with dissolving metallic-painting, fragile display-hinges, fans unbound ... (and needs TLC at any time)
I'd start with a limit 50-70 buck for any of those options.
My favorites for a start-up are either an iMacG3 >=400MHz SlotLoader with FireWire or an iBookG3 Snow (600-800 MHz).