1. ClassiPress is a stand alone theme, so if you have an existing WordPress site, it will replace whatever theme you are using. It should keep existing posts and pages, but how ads look could depend on the current setup (i.e if you're using some other sort of Classifieds theme?). If you're talking about blog posts, these should be fine if you assign them to the blog category when they move across to the new theme.. If the existing posts are "ads" though, they may not appear properly without some tweaking i.e. custom fields would not match up so you'd probably need to do some manual migration work.
2. You can certainly set you payment options this way. Just don't allow ad renewal on the first ads, otherwise renewal will be based on the original settings at the time it was created (i.e. free).
3. By ad space, do you mean actual ads, or sidebar advertising? If it's the latter, this is entirely up to you as income from those ads is managed by you separately.
4. All categories are set by you using the standard WordPress function.
5. This would involve some custom coding, but if you have the skills to do so, then yes.
6. Yes
