Couple of things - if you change your URL to the URL of your existing site with a /exchange after you are going to need users to manually type in the /exchange to access OWA. We found it is easier to create a new entry in DNS like outlook..net so that they don't have to move into subdirectories on their own. It's a user-training thing, they can generally deal with typing something.something.net but something.something.net/something else is often rather confusing.
As far as SSL, you can also create (for free) your own certificate using Certificate Server on an NT 4.0 box. If your users are basically employees they probably won't mind not having a certificate from Verisign and you'll save the $$ and the hassle. It takes about 10 minutes to do on your own box.
Good luck.