I have the page with three levels (more properly, types) of memberships and the button takes the visitor to PayPal. How and when will they get to fill in their information?
In the free version of s2Member, registration occurs after the Customer returns from PayPal; under normal conditions. The Customer clicks your PayPal Button, and upon return from PayPal, they'll be given a chance to register a Username/Password and log in with access to what they purchased.
With s2Member Pro, the Registration/Checkout experience happens ALL on your site, and all checkout steps are consolidated into a single form that lives on your site. You can dress up the form just the way you like, and even accept Visa, MasterCard, Amex, Discover, Maestro, and Solo. Customers will NOT be required to have a PayPal account in order to complete checkout; using the s2Member Pro Module:
http://www.s2member.com/A second question I have relates to making payments via a personal check or cash. Can this plugin handle that? How?
Even in the free version of s2Member, with Standard PayPal integration, Customers can pay through PayPal using an eCheck. With s2Member Pro, using PayPal Pro integration, you will have what's called a "Virtual Terminal" inside your PayPal Pro account; making it possible to receive offline payments.
In either case, ( s2Member, or s2Member Pro ), you can always accept cash, or check by mail. Once you have payment, log into your WP Dashboard, and go to: Users -> Add New. You can assign then one of the s2Member Roles, and even go back and edit their specific account details if you need to. There is also a place on the User Edit panel where you can manually update a PayPal Subscr. ID, or this can also be a PayPal transaction ID.