I prefer to actually mold and cast(make the part) the clients fish when I can however sometimes field care prevents this. This ensures me that I can control the pose and quality. Most taxidermists simply purchase the cast part from a supplier and complete the finish work which is ok and often neccessary too. Purchasing a ready made part can limit quality, size as well as pose. Now to answer the question the only thing needed is the length and girth or weight and for trout and salmon the sex of the fish. Pics are nice to have too but not essential. In my shop skin mounts and replicas are priced the same.