I have a soft spot for the 9600, which can take PCI cards for things like USB or SATA, but their RAM modules are hard to get and their CPUs are very slow. I would also avoid anything older than that, except if you really like the hardware. It’s very difficult to find NuBus expansion cards.

I’d personally say look at models with at least a 400Mhz CPU — PowerBook G3 “Pismo” (I have one of these) or any of the white iBook G3s/G4s (though I’ve heard the keyboards on some of these develop a funky smell with age, which I haven’t verified personally). A Titanium PowerBook G4 may be an option, and they’re historically notable because they basically defined what modern laptops have looked like ever since, but you’ll probably need to do some repair work due to the various issues it’s known for (like hinges that break with time).

iMac G4s are super cool but make sure that the one you’re looking at is one of the models that natively boots OS 9 (some only boot OS X) and that the spring in its screen arm is still in good condition (some have been known to droop with time).