Insert File inserts a file into a container field--which can't be included in the body of an email, it can only be sent as an attachment, so I see no practical way to use that option.
You can set up atable of records where each record uses the same text field to store different email messages. You can then use the Send Mail script step and use the "field" option to specify this text field for the body of the email. Your script would find the correct message record and then send the email.
The exact details will depend on how your database is designed. It's very likely that you'd use either a value list or a "selection" portal to select the message you want and a relationship would link your current record to either the message record, the record with the recipient's email address or both.
Such messages can even be customized to insert text from fields in a record with data about the recipient much like form letter layout might be used to print a form letter with name, and other data about the recipient included in the letter body.