Either a hard Plaster like Crystacast or Jesmonite would be the best for this. You may need to reinforce the cast with fibreglass chopped strand or a fibreglass matt, like Quad-Axial fabric. A porcelain like finish with its smoothness and glossiness on the surface of your cast is going to be somewhat down to the quality and surface of your silicone mould. You can of cause use a clear sealer or varnish (yacht varnish is apparently good) to give the finished surface a shiny, glossy look.