BES on Any exchange box isn't supported, although, it is possible.
for 10 or less users, you might get away with putting your BES installation on a desktop-like box. Yes you would have to purchase a second Windows license for your BES box, that is inevitable.
If you arn't already aware, RIM offers the BlackBerry Enterprise Server Small Business Edition v4.1 for smaller deployments. This is less expensive if you are sure you plan to stay small.