Making a client area in WordPress – Any good tutorials or plugins?

First, for invoicing/billing use the WP Invoice plugin: http://wordpress.org/extend/plugins/wp-invoice/

The “members” plugin may also be useful: http://wordpress.org/extend/plugins/members/

For a custom redirect, to send them to custom pages rather than just the admin panel use: http://wordpress.org/extend/plugins/peters-login-redirect/

If you had them register, greet them with a custom welcome message on the page they log-in to using this code, which should grab their name from their profile:

Welcome <?php echo $user_identity; ?>