I tend to splurge a bit on NAS type components (Drobo, Icy Box) so I can't make a personal recommendation when it comes to the cheaper end of the scale, but I can make a general recommendation when it comes to buying the drives you are going to populate your chosen NAS with: don't buy them from the same place.
Do buy the same model, from the same manufacturer, for RAID purposes. But always try to source each drive from a different place (one from Amazon, one from Newegg for example) even if you have to pay a small premium to do so.
Why? Well, when manufacturing defects occur they tend to occur in batches (get detected, potentially fixed in a later batch). If you get all your drives from the same batch (which is more likely when you order 2 from one place) you may end up with the same flaw in both drives.
That then leads to the "when will my drive fail?" question - if you get different batches you will get slightly different MTBF rates so that unless you are massively unlucky you won't have 2 drives fail at the same time (hopefully giving you time to replace and repair your RAID array in the event of a drive failure).
You can also achieve this by staggering your drive purchases by a few weeks (though be careful of warranty coverage differences), but usually it is relatively easy to find two sellers with similar pricing for a particular model.