Entourage should be able to display HTML mails, but...
Sending HTML mail is criminally difficult using PHP.
Might I suggest you use a library like PHPMailer or Swift (www.swiftmailer.org) to send mails, as they have all of this kind of stuff worked out? It'll make you life a lot easier
It's a good idea to add a 'click here to view online' type link, but you'll need to code that yourself, and you'll also need to upload a copy of the mail onto your webserver where people can view the mail.