You don't need a video card, as rmonahan said, but I'm more on the fence about whether it's even worth buying a low-end GeForce 640 or something if you don't plan on playing games or using 3D apps. The HD 4600 is supported by Chimera, just refer to the install guide for more info, and they work really well.
As for Snow Leopard, you will need the iBoot Haswell ISO on the Downloads page. Just burn it to a CD, boot from it, and install SL, update to 10.6.8, then download Yosemite and make your Unibeast or Clover stick. You won't run SL for very long, and you won't have graphics drivers so expect 1024x768 until you get the Yosemite install started.