The disks are the 'weakest link' when it comes to keeping a busy server working fast and efficiently.
About disk types: SATA disks are cheaper but are slower than SAS disks. Reliability is about the same.
About RAID: All RAID levels (1-10) helps prevent data loss in the event of a hard disk crash. In addition, RAID1 and RAID5 adds slightly better performance than no RAID, and RAID10 adds almost 50% increase in disk performance.