what type / guage wire to install for 12vdc 200w heater?

This is something I don't think anyone here can make a reasonable recommendation for. I would recommend seeking professional assistance from a local upfitter near you who can give you proper advice based on your specific vehicle, intended modification, and specific risk factors involved. When dealing with a heater, you're talking fire hazards depending on where it's located, plus extreme draw on wiring and vehicle resources such as the battery.