Membership and Pay with a Like

I need some advice on which plugins to use. I am developing a site that is going to allow users free access to some online design software if they "Pay with a Like". You already advised me about this plugin so thanks for that. My problem is that I also need to collect an email address and password so that they can come back and sign in to use the software anytime in the future. My thoughts are as follows but I would like your help in advising me what I should use to do the following:-
We are sending an email that will contain a link to our webpage where they will be asked to fill in a registration form (user name and password) and also to Like us on Facebook with a link to do that. When they have done that they will be returned from FB to another page containing the link to the online software. How should I create a login system so that they can come back and use the software as often as they like.

I hope I have not confused you. I am new to WP and would appreciate some guidance.