A well written RFI sent to some vendors should give you an overview of what is available and at what cost.
As you need file level access you should talk to NAS vendors, like Netapp or ENC Isilon. They will certainly have storage boxes for you. You'll have to fit a backup solution to your storage box too, this is work and adds cost.
If you think this may grow, then look at scalability. Not all solutions scale. Also you may end up with millions of files, this may be problemantic to some backup solutions.
I have experience with IBMs Spectrum Scale (GPFS) kit. The cluster filesystem scales nicely and handles lots of files and data with excellent performance. With the recent Elastic Storage components the price per TB is very competitive (5-10x lower than traditional enterprise storage). A half a PB of storage should cost in the $150k range, you'll have to add more for backup and maybe implementation.
For the backup you should look into tape robots. Handling a TB size backup set manually is not fun and will require considerable manpower, when a robot does it mostly unattended. It may also make sense to combine your backup with other backups on site, you may end up be ten imes bigger than everything else combined...