Converting website from html to Wordpress

[vanessac]

Hi!
I am converting a website from standard html (http://chi-analysis.com) to a Wordpress based website (temporarily http://chi-analysis.com/newsite).
I was reading that for security purposes, I should leave the Wordpress site in a subdirectory where I make some changes so the urls don't show the subdirectory name. So, my first question is whether this is correct that I should keep a WP site in a subdirectory.
My second question is whether I should delete all of the old html pages on the root directory (whether or not I use a subdirectory or root for the WP files). My client's concern is that because the old pages are indexed that she will lose traffic because the pages will no longer be there.
Thanks for the help!
Vanessa Cox