Re: Installing as page on already existing site
As you have noticed and mentioned, ClassiPress is a theme in itself, so by default it is not made to be integrated into an existing theme like a plugin. Personally I think there is no reason why you can't still install ClassiPress into a subdirectory of your website and then customize it to match the look of your existing theme. This of course would require customization on your part, but it should be do-able for someone with enough coding knowledge. I am unaware of anyone who has managed to integrate the theme into another, although perhaps they have just never posted here to tell us all about it..
The way the blog feature works is that you create your blog entries from the
wp-admin dashboard, so if you planned on having other users create blog posts, you would need to give them relevant access to the
wp-admin. As for search, there is just the one oveall search function for ads at present, and generally the blog is excluded from this (as blog listings also do not appear in the general ads list). In future there are plans to implement a search filter widget, however I presume it would be ad-related. Again, with a knowledgable coder you could probably create your own search feature for the blog but it would be a customization.