How about looking at the video generation circuitry used in the B&W compact Macs? Schematics are out there, it's a simple, elegant design, built from commodity parts. It would make a reasonably starting point.

Another option is to use one of the various CRT controllers that were used in early video cards and some embedded appliances like word processors and terminals. Or you could try driving an ISA video card.

Seems like a giant project though, you might have better luck starting with something a bit simpler, maybe make a single board computer with just a serial port for a console to start with. If you expose the CPU bus on an expansion connector, you can interface anything you like to it later on.