Even a single SSD should be fine. If you are using the disk for OS as well you may want to go RAID 1 to improve uptime but it isn't absolutely necessary. Unless you are absolutely cash strapped I would always pick an SSD over spinning magnetic disks for an application like this. Having a second drive (slow HDD is fine) to make periodic backups of the blockchain is a good idea. Backups of the blockchain (not to be confused w/ wallet) don't need to be very frequent. Even once a month means cutting your resync time by 99% in the event of blockchain corruption.
As for longevity, yes SSDs do a have finite number of writes but modern ones are well optimized with wear leveling circuitry and additional "hidden" storage space to take over failed cells. The longevity of modern SSD is just short of 1 PB.
http://www.zdnet.com/article/ssd-endurance-death-by-petabyte/ Since the blockchain is mostly appends with very few updates it really is a non-issue.