VIsitors v Free

Still trying to get my head around Membership. Here's what I want:
a. Visitors to be able to visit the site, read posts, see widgets, header, pages. But not be able to post comments. They need some sort of registration to post comments...
b. Free registered users to be able to post comments.
c. Premium members to be able to see protected content.

Is it necessary for me to have a "Free" membership level? Wouldn't this be what used to be "Subscriber" level in WP? On the other hand, if I wanted people to become free members (to streamline the steps toward premium membership), is there a way to ensure they will sign up for the free account instead of the subscriber account?