Where can I find a list of requirements for a ketubah (what it needs say, how it needs to be phrased, and - most importantly - what invalidates it from being a kosher ketubah)?
Chabad have an article on what the ketubah says.
The focal point of the document is the financial compensation due to the wife in the event of the marriage's dissolution through divorce or widowhood. The ketubah even includes provisions which place liens on the husband's different assets.
When a Jewish man marries a Jewish woman he automatically obligates himself to his wife in ten areas; some are Torah mandated and others by rabbinic decree. A number of these obligations are mentioned specifically in the ketubah and others are implied: He must 1) feed his wife; 2) clothe her; and 3) provide her conjugal needs. His estate is obligated to 4) pay her a lump sum in the event that he divorces her or dies before she does. He must 5) pay her medical bills if she falls ill; and 6) ransom her if she is taken hostage. If the wife passes away before the husband, he must 7) pay her burial expenses, and 8) after he dies, her children inherit their mother's ketubah money before the rest of the estate is divided amongst all the heirs. In the event that the husband dies before the wife, 9) she is entitled to live in his home and live off his estate until she dies or remarries, and 10) her daughters, too, are supported by his estate until they marry.
Today, the standard ketubah is a printed form which has blanks for the date and the names of the bride, groom, and witnesses.
The Wikipedia article references a “text of a prenuptial agreement” = ketubah at http://www.rabbis.org/ endorsed by the Rabbinical Council of America. (There is a link on the right hand side of the page under “Commonly Used Forms”).
Chabad have a text too – follow the link.