A blog does have certain advantages such as ease of adding content, However, speaking for myself, I think I would tr to incorporate a blog into my site rather than making a complete switch over. It is hard to argue with top Google rankings.
A few thoughts came to mind as I read your original post.
There's no reason why you can't constantly add new content to a website. You don't need a blog to do that. However, as Desdichata said, a blog can make it easier to frequently update your site.
A blogging platform (such as WordPress) can be used to manage a "static" site. I have several sites I manage with WordPress, but they don't look anything like a traditional blog. For example, sellkidsproducts.com.
And finally, while adding content is probably a great idea, have you tried to determine exactly why you aren't getting more sales? Sometimes we just stumble around in the dark trying to improve things. Taking a little time at first to evaluate what needs to be changed might serve you better.
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