You do not require a tape device to backup to, however it is highly recommended that you store your backups to a meduim where you can take it offsite, for disaster recovery purposes, because at the end of the day
no backup= no recovery
You can backup to the RAID 5 array, by mapping it as a network drive and creating a backup to disk folder, alternatively you can attach the LUN to your server if it's fibre attached.
you only require 1 media server (backup exec), unless you plan to backup each site individually.
I would highly recommend using the Exchange agent and ADRA (Active Directory Recovery Agent)
you will require one of these agents per server that it is being used on i.e 4xDC's = 4x ADRA etc etc.
you will also require a Remote Agent license for each remote (every server other than the media server) that you are backing up, HOWEVER if you purchase an exchange agent you get a remote agent license.