I am planning on making a public guide or a book of sorts that is kind of a larger tutorial that is on it's own website. I am also planning on creating a blog on that site. Would it suit me better with SEO purposes to just make it it's own blog or to make the content of the guide non indexable? What would be the best way to go about this?
Can you transform that guide in a content area or a content hub with an index and its sub-chapters? If so, I'd procede with this, with relevant call to actions to your sales pages.
I discourage you from creating it as a blog post(s), as you have less control of how you can organise things.
See for example this case study: https://samuelschmitt.com/topic-cluster/