1) ClassiPress is a stand alone theme, so it requires its own instance of WordPress to run. So although it cannot be integrated into an existing theme (as in, it will replace any existing theme you may be using) yes, it can be used with an existing theme if you install it in a subdirectory:
i.e.
yourdomain.com <== Your existing homepage
yourdomain.com/classifieds <== Your Classipress installation
2) There are some customers who are using a plugin to allow this at present, however there are plans for this functionality to be built into the theme in the next release (version 3.1)
3) You can of course use standard text/html widget to insert code, or make use of compatible ad banner plugins.
4) Yes, you may post whatever you need to ask here in the pre-sales thread. We'd rather you were 100% sure of your purchase :)