Replacing the dashboard and adding to the current one are both accomplished with plugins. Fortunately, some pretty smart folks have already figured it out for us…
To completely rewrite the Dashboard, you’ll need to create a new page and redirect requests to the built-in dashboard page to your custom one. Fortunately, someone has already figured this out and wrote an excellent post about how to do it over at http://www.wpexplorer.com/how-to-wordpress-custom-dashboard/.
Your second question (adding a small message) also has a good answer already in the Codex (the official WordPress documentation). It’ll take you through the steps to add a new widget to the existing dashboard to display your message.
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