I heartily recommend
Tim McCreight's book, The Complete Metalsmith. In it, he will tell you everything you need to know about purchasing -- or even creating! -- a casting device. He describes the features that you should look for in high-end equipment that you would find in a manufacturing shop, as well as inexpensive methods such as
sand casting and
cuttlefish casting. He also explains all the basics of what alloys to use when casting.