I too recommend a PowerMac G4 tower. They're expandable and customizable.
I've got a mid-2003 "Mirror Drive Door" model with the 1.25 GHz G4 processor, upgraded with 2 GB of RAM, 2 IDE hard drives (an 80 GB and a 200 GB), a new PRAM battery and a USB 2.0 PCI card, and despite the noisy fan (which is why these were often nicknamed the "Wind Tunnel"), it's lots of fun for running old Mac apps, including on Mac OS 9.2.2 and Mac OS X 10.4.11 (this particular G4 was the last Mac that could natively boot into Mac OS 9).
I can even edit standard-definition video on it, even on Mac OS 9! (Here I'm running Adobe Premiere 6.5, the final PowerPC Mac-compatible version.) The rendering/exporting takes a lot longer than on later Macs, but it was a fun little experiment! See the finished video here...
I've got a mid-2003 "Mirror Drive Door" model with the 1.25 GHz G4 processor, upgraded with 2 GB of RAM, 2 IDE hard drives (an 80 GB and a 200 GB), a new PRAM battery and a USB 2.0 PCI card, and despite the noisy fan (which is why these were often nicknamed the "Wind Tunnel"), it's lots of fun for running old Mac apps, including on Mac OS 9.2.2 and Mac OS X 10.4.11 (this particular G4 was the last Mac that could natively boot into Mac OS 9).
I can even edit standard-definition video on it, even on Mac OS 9! (Here I'm running Adobe Premiere 6.5, the final PowerPC Mac-compatible version.) The rendering/exporting takes a lot longer than on later Macs, but it was a fun little experiment! See the finished video here...