I would like to know how to procede to disable a web site from external viewing while overhauling the web site content?.
Password protect the root directory using .htaccess/.htpasswd. Most hosting control panels can create the login mechanisms for you.
Just set up a rewrite rule that redirects any request that's not to the "down" page to the down page.
The one problem with this is OP might want to test the site while it is in production, and he'll be redirected too
Do you want to lose your visitors while you work on your new site? Better setup a testing environment in your computer at home or in a different folder or subdomain in your site and switch when your ready.
If you are using cPanel, rename public_html to anything else (via FTP) and create a new one with a temporary index page. For testing purposes just rename such directories again.