You don't need a NAS to have a wireless network, you just need a wireless router.
However if you want to add a NAS to the network as well then fine.
You can get a NAS as cheap as £100 or as high as the hundreds or thousands of pounds. It depends what you want to do with it and how fast you want it to be.
If you buy a NAS then some come with a hard drive or drives included and some have no hard drives included.
Go for the largest hard drive you can, so 2Tb or 3Tb, but you may want to set up RAID (if your NAS supports it) so the data is mirrored across two hard drives, though you will then need to buy 2 hard drives.