login form

I have a customer that wants to restrict access to specified pages or posts on his website only to his current customers. Everyone else will be able to see all pages, except the restricted pages. Customers who register for the first time would use a cust# or promocode (and maybe address, phone and other info., but thereafter would only need a username and password. He also has a concern that a registered user could give login information to unauthorized users, or that a bot could create multiple, bogus logins.

I presume that registration and login information would be stored in the database. Could this information be accessed by me or my customer.

Can anyone recommend a plugin or articles that me to educate myself?