Actually none of these options are good. You may buy already written articles, but you'll need to insert your keywords into it if a copywriter has not done this before.
Rewriting articles will do no good either as Google indicates such content alterations and you may get penalized.
From my experience I can say that the best option is to hire a good copywriter, provide him/ her with your project details and specifications and get your article/ post ready.
Or you may create content for your blog yourself as you know your field well and if your grammar is OK.