How do I find a writer with a specific area of expertise?

I would have your experts write the articles and then hire an SEO to tweak the articles. A good SEO should have enough writing ability to not mutilate your articles.

When I was hiring writers (I am the SEO expert in my case) I used to ask what niche(s) they write in and very few would give me an answer, they would always ask what I needed. I wouldn't tell them because I knew they would just say they could do it. If they had expertise in certain subjects they should have been able to tell me. 99% of them could not or would not. I didn't hire them because of that.

As far as SEO being worth it or not. Why not do it? If you are going to have articles they may as well be optimized as much as possible. It can only help you.