i dont want to spend any more than about £500 for the NAS only, without the hdd, actually, now i can just get a 3 bay nas, as 4tb hdd are availiable.
Consider building one if you have the time and inclination. You don't need much CPU power at all, 4GB or so of memory, onboard video and Ethernet.
FreeNAS is excellent for the job, and if you want a really robust storage system look at ZFS and consider raidz. If you have enough onboard ports on a nice little inexpensive mini-ATX board you are good, but you can always add an IBM M1015 host bus adapter (not a raid card) in the PCIe x16 slot to add 8 more drives for around $100.
i dont want to spend any more than about for the NAS only, without the hdd, actually, now i can just get a 3 bay nas, as 4tb hdd are availiable.
Do you consider Safety side of the build ?
On 3 bay storage with 3 x 4 TB you can have RAID 5 -> 8 TB storage and 4 TB for redundancy
My point is more disk = better redundancy and performance also ,,,
I personaly will go for 4 bay with 4 x 3TB in RAID5 = 9 TB + 3TB for redundancy
or 4 x 4TB in RAID5 = 12 TB + 4TB for redundancy