It's possible to customize the theme, but whether it's "easy" would depend on your level of coding knowledge, and considering most people haven't implemented an advanced search themselves, would indicate it isn't an easy thing to do. The good news though, is that the advanced search functionality is already planned for the
ClassiPress version 3.1 release which will allow you to search by selected custom fields.
There is no mobile version of the theme, but I understand some have had success with mobile related plugins, however I also believe this involves some amount of tweaking to get it working. You are welcome to check out the
ClassiPress demo for yourself to see how it looks without modification or plugins on these devices...
