Is this a legitimate way to post? It feels a bit like plagiarism. Do I need permission to link another's post as the main topic of one of my posts?
That's a great question, Idahoe, and it's something that a lot of new bloggers have questions on. Unfortunately, a lot of them get it wrong. Here's the deal.
Linking to someone else's blog is always acceptable. You don't need their permission to do it, and if they know anything about blogging, they'll be grateful for the link.
Quoting other people's work is where it gets a little bit trickier, but there are still easy-to-understand guidelines that you can follow.
Quoting someone's entire article and posting it on your site is a copyright violation. Even if it is correctly attributed and linked, if you do it without their express permission, it's still illegal.
So don't do that.
Quoting a few excerpts, however, is fine so long as you give credit for the quotes to the author, and link to their original article. I like to use the /blockquote tag to clearly delineate what is their work and what are my comments. The amount of text that you can quote is a gray area, but you can safely use your own judgment on that and decide for yourself. Ideally, you would quote enough to pique your reader's interest, but not so much that they can practically read the whole article on your site. And no, you don't need their permission to quote their article and link to them. You can always send them an email letting them know you linked to them, but nearly all bloggers will see the link come in as a trackback and will know from that.
Hope that helps.