Buying two extra drives makes sense. I just wish I didn't have to keep telling our provider she needs to spend more money on more hardware for a server she already spent a lot of money on less than a year ago.
If Dell is providing support, then my guess is that the drive(s) that is failing should be covered under warranty. Having at least one spare drive is just good process for just such an eventuality. Dell should not be charging you to replace them.
Sadly, I have had two different clients with Dell machines have hardware failures at the 12-14 month point recently, and both were repaired under warranty, but it is still tremendously disrupting.