1) Yes, you can use eCart with subscription based sites.
2) The registration would be doe using security assist. the way this would work is that you would have user level access, a basic registration that would allow them to log into the store, select the option to buy a premium registration, then checkout.
there are tutorials in the archived documentation section of the security assist support page for user levels, and documentation on the eCart page for adding registration to the store checkout.
once the payment is completed, the users level would be upgraded from basic to premium to allow them access to the paid for content.