Choose an iMac with USB, and then get any of many USB/serial adapters. Keyspan springs readily to mind, and they have been at it since at least 1998. The adapters fetch up to USD60 new, and hold their value well in the used market, although you could be lucky. Usually the adapters will connect one USB to two serial ports.

The Chooser remains (up to OS 9) effective for choice of serial device.

de