1) ClassiPress includes options for you to set up subscriptions/membership packages which are for an unlimited amount of ads for set price and duration, so yes you could essentially set up the packages you describe. The options can be seen on the
ClassiPress admin demo here.
2) Yes, the theme will keep track of membership expiry
3) Yes, they can select from the packages which you create
4) The theme does not have the option to limit posting for subscriptions, so any package you set up is for unlimited ads
5) If they know to look at the source code and you don't remove any footer credits then yes. You would have to make your own modifications in order to hide this kind of information. Thanks.