Welcome! As others have said, it's best to first build something simple and work your way up. Things you don't understand at present will become more clear.

For the crystal, it looks like you should be able to find one at 21.47727 MHZ and then divide by two.

If you don't have an electronics ...

I would recommend searching for the CRT Controller Handbook by Gerry Kane. You can find a download for certain. It doesn’t cover the 9918 controller, but several others and covers the basics of video generation very well. This would give you a solid background on video and perhaps persuade you to a ...

I'm working on designing my first homebrew computer, and I have a good handle on most every component- except for the VDP.

I could live without video and go for serial/term output, but my goal is to have composite output. Additionally, I'm trying to avoid using an FPGA, AVR, Gameduino, or Propeller ...