Going to Mexico to find a doctor on a lark sounds like trouble. There is a good chance you'll find a clinician who may have no surgical experience, let alone castration, willing to wack your balls off, pat you on the bum and send you back across the border.
Castration is not so common that residents in med school would ever gain a proficiency. Depending on if a person wants the scrotum removed, my preferance, the amount of experience may be nonexistant for a doctor. Castration for prostsate CA would not involve removal of the sack. The scrotum being highly vascular with muscle and nerves, would likely be much more difficult to remove than just the testicles. A cutter who uses multiple sutures to close the incision after removal of the scrotum would be desirable. Again, going to Mexico, you may have some guy cut your sack off and staple you back up! What would be left is a nasty scar.
chill