Hi Jack,
If the PC you are planning to use as the NAS already has an operating system installed such as Windows, then to avail it offering network shared storage, (indeed, it probably does already,) then all you need to do is install the extras discs and create the "shares" (as Windows calls them.)
Windows does have some "limits" as to how big a disc it can handle, so it would be worth checking which version of Windows is in situ and thence what the biggest discs are it can handle. Indeed, the motherboard and chipset of the PC might have limits too, so it would also be worth checking there, especially if it's old.
Serving files is a pretty low "horsepower" activity for a SOHO site (ie with few users,) so you don't need mega CPU's or RAM and you bearly need a video card at all, so practially anything will do (the incumbent will be just fine.) In olden days, file servers didn't do graphics at all - they had the most basic 80x25 black and white character displays...!
Unless you particularly want to "play" with a NAS operating system (OS,) or you want some feature that a NAS OS offers that Windows doesn't, there's no real need to change it.
There's nothing particularly "magic" about a NAS - it's just a very simply computer with bearly enough resource to serve files over a network. That made them cheap and simple. A lot of NAS OS's are stripped down Linux builds.
Over time, there's been a lot of "function creep" in the NAS market place as vendors have "added back in" a lot of the features that were omitted originally in order to make NAS's "simple" (and cheap.) Such as media server, backup servers, and so forth. Theses days there's not much to choose between a "full fat" OS and a NAS OS. But if one remembers NAS's origin, one sees why they are as they are.
So in summary, if you are repurposing old hardware to provide network shared files, especially if that hardware already has an operating system installed, you probably don't need to bother installing a NAS OS. Having checked out any potential size limits, just add disc, make shares, connect to network.
About the only other hardware upgrade you might want to contemplate is to replace a 10/100mbps Network Interface Card (NIC) with a 1000mbps (Gigabit) NIC is your router has any Gigabit ports. If the MOBO NIC is already Gigabit, then your are already there.
It's worth pointing out that a repurposed "desktop" PC might use a bit more electricity over the course of a year that dedicated NAS hardware, but that would require a bit of forensic modelling to be sure.
You mentioned using "online office" - are you talking about remotely accessing apps running on the NAS/PC...? (It's possible, I just want to be clear about what you mean.)
Please don't be discouraged if you have any subsequent questions, post back - we're a pretty friendly bunch here.