When choosing cremation, is it necessary to purchase a casket and embalm the body? 

If the body is present for the funeral liturgy, with cremation to follow, you may rent a casket or purchase a special cremation casket that will later be consumed in the cremation process.  In situations where the body cannot be present for the funeral liturgy, a casket would not be necessary nor would the body need to be embalmed. However, the crematorium requires that the body be placed in a combustible, outer container prior to entry into the cremation chamber.