I disagree with
add your sitemap in this page. - this doesn't make any sense. You should link to sitemap instead. Because if your site has 10000 internal pages it would be not normal to put 10000 links to just one page.
Additionally, if you set-up a 404 page it will return the code 200. Said in a simple way, non-existing page will return "success". But this is OK, 404 pages are necessary for every larger project

. Good post at all.