My first Mac

Beige G3's are OK, and still have SCSI and serial ports. I have a gaggle of G3 powerbooks that I use at CHM for various recovery projects. Blue and White machines switched to USB and dumped SCSI and serial, which make them less interesting. If you want that sort of machine, get 2nd generation or later Grey G4. Avoid the last couple of generations of G4's, they had cooling problems... Putting a 1+GHz CPU upgrade into the earlier G4s makes a pretty decent system. I'm using one as my main machine at CHM. Probably not worth messing with. You aren't going to be happy with the performance of a first generation PowerMac. If you really want a shell in the OS 9 environment, there's MPW. I remember someone playing with getting a bare-metal OS going on PPC Macs at one point with a CLI. Don't know how far they got. This is sort of interesting if you wanted to do some low level turn the interrupts off kind of programming on the machine, since you can't actually turn interrupts off completely in the OS 9 environment.