Well In your case If i was you I would go directly to ZFS simply because of two things:
One it can work with any type of drive aslong as it's the same size as the others (give or take a few meg).
Two it also allows you to set profiles so that you do not have the deletion permissions except on a single account.
To get the best out of this setup I would suggest also completely diving in now, and turning all your PC's into SSD machines and a server hold all your data, then all you need to install is samba for windows interaction and your sorted for data storage.
As an example all my machines are now holding either a small 128GB SSD (which are about 60 quid now samsung 840 EVO is awesome), my TV is connected also to the network so it can read the Plex media server (so i don't need a HTPC). Along with this all the data is stored on 3 different HDD's all 2TB in a RaidZ1 (which is raid 5 with data correction), then the area's of extreme importance are transferred via snapshot to an external backup drive.
What I like most about ZFS is "scrub" it allows the system to scan through the drive looking for error's in the data if something becomes corrupt it is replaced out from the parity storage bit (which you could set on a schedule).
For you this would involve another machine but as your considering purchasing a NAS your already spending alot of money anyway.
So to save cash I would do is use your two current HD's in a new server to give you an idea this is kind of what I would expect you to pay for this kind of server:
MB: £100 Z87
CPU: Any cheap CPU I'm a Intel Fan atm so mine cost me £40 quid for the lowest end 1150 socket
RAM: 8GB of RAM (That's for ZFS, ZFS needs lots of memory 1GB PER TB is best) £80
HD: Buy another WD Red 2TB and turn those three into ur main network storage £70 (you might have to buy another drive just to be able to transfer this stuff unless you know someone else with the storage, so like you said using the WD green as the end backup).
SSD: Two 840 EVO's £140 for the two and using the 500GB drive as the server OS
That's a £500 quid purchase so that isn't cheap and that's not even including the other parts needed. This could end up being about £650 quid at the end with a case and good powersupply (high efficiency is what your looking for so platnium is the kind of level your looking at).
However this is the most expensive route, the other route you can go is to buy a cheapo PC for a £100 quid and stick a GB ethernet card inside, aslong as it has space for your harddrives you would be fine (then head your way into debian server fun or whatever your fav linux os is). Then buy another 2TB WD Red and place it inside and create yourself a ZFS Mirror instead (gives you the benefit of data correction at the expensive of losing a whole drive to parity).
However as your retired you might want bang for buck more than anything so you'll pay more for this setup in the long run in electricity, it just all depends on your budget really and if you think your ever going to need more storage.
Also to Note if your TV is new you can just use your HTPC as the server (and place it somewhere away from your TV) it would save you buying another PC and use it instead as a media streamer. But again this all depends If the TV you are using is new, the big giveaway if it does support DNLA is the ethernet port on the back.
Hope this helps long post but quite hard to explain in detail otherwise.