I agree with what Bert said as well with one modification.
Third, as just mentioned, Dell support is excellent. Bad hard drive at 3 pm. Get ready for a new one at 10 am the next morning.
You probably want to keep an extra hard drive on site or get a "hot spare" in the server. In case one of the hard drives fails, you can reconstruct the array and continue business as usual and keep the extra drive that Dell sends you as a new backup. If the failure occurs when you open the office, you probably wouldn't want to wait until the next day to be able to use the Server. This is the main reason people recommend RAID 10 over RAID 5, the rebuild times for RAID 10 is like 5 times faster.