Wordpress Plugin - add an 'I Agree' checkbox to the login page.

General information for the website: The website is already designed, it has a private area for members to login to that needs an agreeable checkbox.
Description of requirements/features: I'm designing a website for a client who needs a special function that I cannot do because I am a designer with no apt for coding functions.

The need is for an 'I Agree to the NDA' checkbox that users need to check in order to login. There should be a link to the page or pdf of the agreement NDA in a new tab.

The users are added manually, not through registerting themselvs. So when they come to the page to login, they have their details to do so, but at this step they need to click the 'I Agree to the NDA' before logging in.

1. Their choice should be remembered for future logins.

2. It needs to throw up an error message if the 'I agree to the NDA' checkbox was not checked not checked. Error message, "you need to agree to the NDA before logging in to the private area"

3. Not essential, but an admin options page would be good to change the messages or the link to the NDA.

4. please deliver as a plugin zip

Not looking for a power-plugin here, just the basic function of ticking the box to be able to login.

I'd like to keep it a super simple solution just to get the function working, but of course, I have no idea what's involved so you tell me how long it's likley to take.

Please try not to get technical when communicating about this, I probably won't understand what you're talking about :)
Extra notes: My website uses the Divi theme from Elegant themes.

This is the Divi themes login page module used on one of my website pages: http://www.elegantthemes.com/gallery/divi/documentation/login/

THE ATTACHED IMAGE SHOWS the two places for logging in.

The sequence goes like this, when on the member login page and either the wrong username password or the i agree checkbox is unchecked, it will as standard divert to the default wordpress login page, and throw up the normal error, or if they have not checked the i agree box, the custom error message.

Both the member login area using the Divi theme login module needs to have the i agree box and so does the default wordpress login page.

Please make any suggestions if you think there is a simpler, more standard or easyer way for you to accomplish this. It is important though for the client to have a member login area as part of the website page which is why I'm using the login module provided by the Divi theme.

Please advice on any possible future problems as this plugin gets older, what's the possiblity of it stopping working and does that mean we need to come back to you to fix it?

Please advice on how to see a list of who has logged in and when, so we know who has agreed to the NDA and has successfully signed into the private area.