Dielectric power handling.Can PCB support upto KW's of power

You can search in Rogers or Murawa, it should be Ceramic with Er=9.6? thickness=1mm?

Is almost impossible to carry a 10kW RF signal using microstrip transmission lines. You need special coax cables, or waveguides if frequency is higher.

the dielectric loss will dominat the temperature rise in the PCB substrate so ideally a very low loss substrate can support very high power. However if you consider the microstrip line loss and component loss(like capacitor), they will heat the substrate also. So the best way is to run a thermal simulation before you start a proto build.