Fully custom registration page

I have searched everywhere for a solution to this problem! Perhaps my Googling skills need some more practice ??

I would like to implement a fully custom user registration page on a WordPress site. I would like to use custom fields which would include radio buttons and option fields. It needs to include some sort of captcha system, form validation and insert everything into a database.

After the user has entered the required information to register, the next part (either to a separate PHP page or through AJAX) would have to send a double opt-in activation email to the user while inserting a few extra fields which would enable the user to invite friends. Once that is submitted, it would have to send those emails.

All it has to do is allow users to register and login to write comments, receive a newsletter and be pre-registered to a forum which would be implemented later.

Over and above that, these have to have custom CSS and include the site’s header, footer and possibly sidebar!

I have achieved this on a non-Wordpress site but am struggling to get information on how to do this within WordPress. It seems a lot more complicated. A plugin it seems, won’t do.