If you ever want PCI compliance for any of those sites, you will HAVE to have an SSL certificate installed. As far as as getting it to run over HTTPS, you don’t need a plugin, just open your .htaccess file and force https on all pages:
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
Just remember to restart Apache when you are done ??