Since you're hiring a freelancer I would suggest:
• Scout for someone who's had background in the same/similar domains. That will make things more easy.
• Judge them on - past work, timeliness, content not requiring grammatical edits, general flow of the pieces they have written so far. Goes without saying you'll have to work with them on a couple of blogs/articles to get a sense else rely on gut or portfolio.
• In my experience, quality freelancers charge on per article basis and decide on pricing basis the depth of articles. The ones pricing on per word basis won't make the cut for your requirement. You can work with them for a couple of articles and then increase the scope.