Why do you want to do this all over email? What you want to do sounds like it's going to be incredibly tricky, especially the part where people update the text in an email and the server is meant to make sense of that. You could force the email to be in plain text, but if the end user decides to change the layout even a bit then it could break in your script, and a lot of email clients forcibly insert new lines into plain text emails where lines go beyond a character limit (typically 80 characters)
This could be far more easily accomplished with a web-based form rather than email. After all, the user still needs an Internet connection for both, and you won't have to spend a lot of time messing around with parsing raw emails in PHP.