Below are a few items you need to determine before hiring any proofreader, copyeditor, editor, or any freelancer (ghostwriter, illustrator, graphic designer, web designer, etc.).
(Actually, this writer probably puts it much more succintly. Check out this Writers Digest article on what to look for in an editor by Tanya Egan Gibson.)
What are you, the writer, responsible for? What is the freelancer responsible for?
What, specifically, are you looking for?