CiviSites Professional is for people who want to run CiviCRM for managing their contacts using the full range of capabilities. CiviMail is enabled and so is CiviCase along with everything else. You will also get access to our support ticketing system so we can help you out if you get stuck or you want advice.
There is no limit on the number of contacts or users and you can run this as a back office product or a public facing website. There is a limit of 5,000 emails a month through CiviMail. Please contact us if you plan to be sending more than this.