Well, it depends on a lot of things:
How much storage space do you need? You mentioned NAS, so I'm assuming that you have more data you want to store centrally besides just a QB data file.
What is your budget?
Are you comfortable managing Windows Servers?
You could get a basic server (T110 II) for less than $2000.
You could get a "good" server (for what it sounds like it would be used for), like a T320, with high-performance storage, for $3000-4000.