For doing large amount of data storage in house you have issues with redundancy and depreciation of hardware, it also does not resolve the problem that if there was a fire as your backups would also be lost as well as your entire systems.
You should imho be looking at off site storage preferably something running a fibre channel DAS.
Any decent IT service provider will be able to offer a scalable solution which is cost effective as you do not pay for capacity you are not using and the hardware investment and depreciation costs, not to mention administration costs are not assumed by your company.
The other advantage of using a IT service provider is that they will be able to offer different types of backups and store these for over a definite period of time, thus giving you more than one viable restore option or backup to do this effectively, securely and in a cost effective manner in house is simply un-viable for most business.