Think the key word here is datacentre, and nothing scares a customer more that hearing that the future of his company is sitting firmly with a backup application he or any of his staff ever heard of.
If you are offering this to your customers as a business, make sure that you select a product that can do EVERYTHING - don't waste resources training your local guys to support multiple backup solutions. There are lots of great backup software out there that will do everything you ask of it - in exchange for hard currency, so choosing one might be tricky.
Now there are certain advantages of matching your storage vendor with your backups solution - you'll find that you can cut through some of the red-tape and get straight to the point, so if you are using IBM storage have a keen look at TSM, EMC(&Dell)/Legato, HP/DataProtector - they will more than likely be able to assist you with this project and offer tips and tricks to deploy and scale this solution.
There are quite a few challenges involved in deploying a large solution like this and truth be told if I were in your shoes I would probably stay away from this if at all possible. If that isn's an option then I would start by looking at some of the major applications on the marked - NetBackup, Legato, TSM, CommVault etc. and see what they can offer in terms of deployment, support, training, licensing, price etc.
Once you have found a scalable piece of backup software, you'll really get into trouble - what hardware will be required? If this will be an offering to existing customers you will need a very god estimate on the number of clients involved - and in particular the amount of data in question! You don't want to offer a solution that is slow from day one, and no point in getting a silo to backup 5 servers and a mailbox.
If you plan to go on with this project keep us updated (and see if you can get more details on the amount of data you're looking at).
Best of luck,
Jesper