These kinds of search/filter features can certainly be customized by someone with enough coding knowledge, and I have seen examples of these before. We will be seeing the introduction of a search filter widget in a future release of ClassiPress, however this is not currently available.
Yes, you can definitely customize the theme, and with the latest release it is now even easier to create a child theme, enabling you to upgrade without losing all the work you do to customize in the first place.