Well, you have your construction idea right...2 parts is good, and you can attach the top and bottom once cast. The top will be a pretty easy molding job, easiest being to build a box around your sculpt and pour the silicone in.
For the piece that goes around your arm, I personally would go with a brushable silicone. Brush it over the whole thing, covering it completely, including the sections that you will be cutting out in order to put your arms through. Try and include some 'keys' on the outside of the mold that will allow you to fit the mother mold around it in exactly the same way every time.
Once your silicone is thick enough, you'll need to make the mother mold, this will need to be 2 piece. Use clay to define a seam line around the mold. You can place indentations in the clay if you like that will act as keys in the mother mold, to ensure that the two halves always fit together the same way. Do one half of the mother mold using whatever material you've picked. Once that cures, remove the clay and do the other half, making sure to use something to separate the two halves of the mother mold so they don't stick together. If you're using plaster or similar then vaseline does a good job, same with fiberglass. Do a test sample first to see how much you need to use, because I guarantee you nothing is as frustrating as your mother mold sticking together. Other than getting everything off and finding your mold hasn't worked for some reason.
Before you remove the mother mold, think about how you're going to hold the two pieces together. I drill holes in mine before I ever take it apart, and fit nuts and bolts to hold it together securely.
Now you can take it apart and demold your sculpt. Depending on the silicone you used, you may simply be able to cut a hole in one end a couple of inches in diameter and peel it off like a glove (Smooth-ons dragonskin is stretchy and strong enough to do this). More likely, you will need to take a very sharp knife and cut along the mold. You can either split it completely in half, or just partially split it. Once demolded, put everything back together and you're ready to cast! You will be able to pour resin into the mold using the hole you cut in one end of the mold. Because you haven't cut a hole in the other end, you can just slush the resin around inside until it cures.
Hope that helps you some...
Oh yes, and I thoroughly recommend the molding tutorials on smooth-ons website. They may not be exactly the same as your application, but they have a lot of pictures and will give you a great idea of how to do things that I've mentioned here. I know for me it helps immensely to see pictures rather than read about it.
