Conditional Email Notifications

I'm trying to figure out how to give users a front end option via a Buddypress profile tag (and possibly a widget too), to setup conditional email notifications (ebay style), based on certain criteria, ie if they are looking for a red bike in their home town and someone enters a post that matches this. they will have the option to get notified by email (either instantly or daily).

I'd like to request this as a future feature addition but it's also something I really need now, so can anyone suggest a way this might be possible please?