Need to upgrade website without public seeing it

This may seem like a simplistic question, but I have never done it before.

I have a wordpress website/blog that had been around for almost four years and has a lot of traffic. The posts and comments are well over 20,000 entries, so I need to totally overhaul the entire site, change the theme and pare it down.

How do I put up a holder type of page that maybe says 'Under Renovation' or something similar, but that will still allow me to make all the changes in the site without anyone seeing the mess?

Also, if I delete/hide pages and comments how do I avoid getting a google 404 crawl error?

Thanks so much