I would look for a Mac with a 120MHz+ processor (603 and up). All the old Power Macs have SCSI ports which support modern drives either internally or externally - the problem is that 20GB SCSI drives are expensive. You could get a SCSI-IDE converter, though. There also might be NuBus or PCI IDE cards, although I would be surprised if the drivers for them ran on anything other than OS <=9.