The
WP Custom Search plugin is total rubbish - we've tested it.
For anyone running, or proposing to run, a classified directory, then filtering is a must have feature but it's not doable if you rely on using custom fields to store hierarchical values such as country, region, city etc.
I recommend that you use categories exclusively for this because you can go deep and they are very easy to manage.
This plugin does the job very well of handling filtering but will not work with CP:
http://scribu.net/wordpress/query-multiple-taxonomies