pretty old guide, but it's still relevant:
https://www.blackhatworld.com/seo/guide-full-example-of-how-to-do-on-page-seo.1251425/
they give your site relevancy (google, being the idiots they are, they only want to see "business" type of websites in their retarded SERPs, even if we are talking about old fashioned, personal blogs, google still wants to see legal pages in order to consider your site as being legit, typical retarded corporate mentality, what can you do?)
So yeah, it's good to have those pages, but there are many online generators for legal pages, just google them and use those to generate the pages for your site (just don't forget to replace the placeholders with your actual site / company's information before publishing them. I've always used Privacy Policy online generators and never had issues ranking my sites with these fake-ass templates)
hmm, that's a tough (because it's broad) question...
Probably you want to pay attention to using a good domain name. If you buy an expired domain (that was used for a different niche to what you intend to use it for) and now you're changing its niche you're already starting with a (possibly non-recoverable) handicap because I've heard (can't confirm, though) that google - in their lunacy - have started blacklisting domains they don't like. And they never like what they can't control, so probably that many 100s of millions of domains are already on their idiotic blacklist, and if the domain you're going to use for your blog is on their blacklist you'll probably never rank no matter what you do...
If you don't know how to check expired domains properly, either buy 1 from
@Nargil (who is guaranteed to be clean), or simply use a fresh domain. But when I say fresh I do mean that you have to make sure it's fresh and never used, otherwise it's probably already blacklisted by google if it's so easily available for newbies to snatch.
Probably that's one of the most important things I can think of to watch out for, but like I said, your question is too broad and can encompass a lot of stuff, so you need to be more specific with what your concerns are in order to be able to give better replies.