The only time I’ve fooled with a paid theme was to find out its secrets… Or one of my clients had a paid theme installed already.
As to SEO? The worst, ugliest, most featureless theme can have the best site SEO wise using it. The theme doesn’t get a vote on the SEO for the most part.
My take on SEO? Content is King!!!
And if you need some SEO help then consider Yoast SEO.
But if you have lot’s of great content, that’s hard to find elsewhere, then the search engines are going to pay attention. Else, you’re just playing in the same sandbox with the rest of the class. Nothing wrong with that but you’ll need to play harder.
The theme serves to present your content, which is your message, in the best possible light. So when looking for a theme, look to the WordPress repository’s themes as they are free or free with a premium version. The Premium version is quite often a good value on an already nice theme.
If you see a site you like the looks of then try to find out what theme is being used there.
If it’s a premium theme then consider the price vs the probable value. There’s a lot of nice premium themes out there.
But the content makes the site… not the theme! Not the SEO even… and not the plugins.