Hey Robert,
I am a professional mold maker. I do mostly industrial molds, but have been casting sculptures for over twenty years. I highly recommend Smooth-On for you supplies. I work with many others, but they are great with customer service and their web tutorials are the best. The big question in what material to buy is, how many cast are you planning to make. Tin cured or Platnum cured. I cast in Platnum Silicone and do it in a pressure pot, so there is no air bubbles in the rubber. They also make a prototype rubber that is a one and done type of rubber.
The type of mold you are doing is the most easiest. It is a one part open face. All you have to do is build a box around it, at least 1/4 inch thick. Pressurizing gives you the best detail in the rubber. If you don't have a pressure pot, you can pour it in. You need a small steady stream and pour about 2 feet above the mold. A small stream will give you less chance of air bubbles. Pour in one spot and let it fill in. Make sure that your surface is level.
I recommend smooth cast 320 for the resin and you can just paint the casts. You can get brown pigments to tint the resin, then you would want smooth cast 325.
I am new to the forums, but have been doing this for years. If you have any questions feel free to message me