You can customize the frontpage to be a search only but it can't be done via the backend. You need to modify your index.php or create a new home.php so it would require some coding skills. You could also ask some professionals who are offering modification service.
The current search offers search in category and some advanced features, such as an autodectect feature for keywords. Some more features will be available in classipress 3.1
In classipress 3.1 new users can create a password on account setup or login with their facebook account. You can see the full list of features that will be available here:
http://appthemes.com/blog/classipress-3-1-preview/
P.S. thinking of it ... what you could do regarding a frontpage without ads is creating a normal page and setting it as startpage in your wordpress settings. Here you could write some introducing text and put the search widget in the sidebar. You won't need any coding for this. Could this work for you?
Last edited by bluecafe; December 7th, 2010 at 04:57 PM.