Login System with Paypal payment option

What I need is login system + paypal as payment processor. The system should be able to register users, after that to send them to paypal to pay for a service and after that return them to the service page. The system should check if the user has paid and if so to automatically activate (or activation by administrator or both) his/her account. Also this process should happen WITHOUT email address. Also the system should be able to check (on login) if the user has already logged into it, and if so the second user to be given an error and the option to pay for his account to be able to use the service. Optional - The system may have feature to track from where the user has come, or the registration form to have a list of sites from which the user could choose. If the form has that feature (list of sites) the system should be able to generate code, and with that code pasted on the partners site, when the user clicks it and comes to the registration form, the site of the partner to be automatically chosen and the field to be inactive. The Database should have at least 3 fields - Username; Password; Referer Also it's very important that the system could be integrated with ease to different layouts.