1. You can set an expiry date, even 365 days if you want, but you do have to put something in. You can of course also choose not to prune your ads, so even if they expire they won't be removed from your site. You could do some minor customization to hide the expiry date field anyway, so to a customer they wouldn't see any reference to it.
2. The comments section is just standard WordPress functionality.
3. I'm not aware of any specific plugin for this which is compatible with ClassiPress. If do you find one that is compatible with custom post types it may work, but we cannot guarantee this.