I am of the opinion that it really depends entirely upon what your business-needs are. It begins, of course, with what sort of search capability you intend to provide in your product ... what options you provide and how those options may (or may not) be used together. This, and your database schema, defines what the search strategy or strategies may consist of; what makes sense. And this, in turn, governs the implementation.
The Catalyst framework by-design does not impose any strictures upon how you “must” implement the Model layer. TMTOWTDI™.
Obviously, one thing that must always be in the front of your mind when dealing with user-generated queries is that you must not allow the user to query for the entire world. You must set reasonable limits not only on what the user may ask for, but how much. In your logic, you must know, and know very well, what each query possibility ultimately leads to.