What were the legal requirements for the messiah?

Answer 1

Well, from my understanding, a Messiah is a special being, sent by God; so no normal human being can become a Messiah.

In case you mean how to recognize a Messiah, the issue is rather complex. One sign is given in The Bible, where it says (when talking about false prophets), something like: You shall recognize them by their fruits.

Answer 2

Jewish answer:

The Jewish requirements of the messiah are:

* Build the Third Temple (Ezekiel 37:26-28)

* Gather all Jews back to the Land of Israel (Isaiah 43:5-6)

* Usher in an era of world peace, and end all hatred, oppression, suffering and disease. As it says: "Nation shall not lift up sword against nation, neither shall man learn war any more." (Isaiah 2:4)

* Spread universal knowledge of the God of Israel, which will unite humanity as one. As it says: "God will be King over all the world. On that day, God will be One and His Name will be One." (Zechariah 14:9)

* The messiah must be descended on his father's side from King David (Genesis 49:10 and Isaiah 11:1)

* The messiah will lead the Jewish people to full Torah-observance. The Torah states that all of its mitzvot (commands) remain binding forever.