Assistance building a backup server

[Alex Regan]

Hi,
I have a fedora20 system acting as a backup server, and I've exceeded 
its capacity. I'd like to build a bigger one, probably using fedora21.

I currently have a 3TB backup system using five 1TB disks in RAID5. 
Restore times in case of disk failure are already exceedingly long, so 
I'd like to consider another method of providing redundancy, and would 
like suggestions.

I'd like to have 6TB of usable space using 2TB disks.

Is ext4 still best for this?

Some RAID variant or is there something better?

Are there any NAS projects that may be beneficial?

Thanks for any ideas.
Alex