This is not an atypical situation. I would say any second or third year student would be qualified, but some will have more knowledge of your specific areas depending on which classes that student has taken. Most of the second and third year classes are electives, and a student that has taken consumer law, for example, will be much more efficient working on a consumer law case (because she will have already studied the foundational cases and issues).
I would expect to see a pay range somewhere between $10 and $20 per hour for this type of job. (This is based on my experience in Boston, the range might be different in other areas of the country.)
I'd say that prior work experience can be very important depending on the industry. You might work better with someone who knows something about the way business is done in your industry.
Take note of the academic schedule. Students can sometimes have varied availability.
Lexis and Westlaw do not come free with the student, and that's an important consideration. You should either provide access to the student or remind them that they should use only library and free internet resources in their capacity as an employee.
In my opinion, law firms overemphasize grades. Remember that grades are generally determined solely by a final exam. Look instead at research related classes such as Legal Writing and seminar classes (which usually determine a grade by a term paper instead of an exam). Request a transcript for grade information.
To find a student, I'd suggest placing job listings with the career offices of various law schools. I think most law students turn to the career office first when looking for employment.