I'd recommend RAID5 or at least RAID1 drives for your first backup server. Check out Rsync for how to perform your backups
Raid alone will be not enough to provide backup services. Raid is not backup, its just for improving disks performance and for service continuation in case of disk failure.
There still are great chances of losing data with RAID as 70% of data lose are not physical problems, but software or data corruption. I cannot imagine a backup provider that tells their clients they just lost their backup data and they are suppose to be the backup.
He will not only need software, server and raid, but also a second server to replicate data or save the data offline. A backup of the backup.
Most providers dont do this, as it increases their costs, but if someone hires a backup service, he expects the backup provider do the same service they provide. Backups!!!