If you're out for the maximum bang for the buck, you might want to check out used SAS drives which are generally a lot cheaper than SATA equivalents on the second hand market. You'll need to factor in the cost of an HBA card, but you can get these for around £10 or so from the auction site. Not sure how well they'd play with a microserver as I don't own one - I guess space for the card and airflow might be a consideration here.
I bought 4x 6TB 7200rpm Dell-branded Seagate SAS drives for less than £100 a few months ago - they all had fewer than 100 hours power on time and a clean SMART bill of health (not a foolproof guarantee, admittedly). They haven't missed a beat so far, and don't seem to run any noisier or hotter than my other SATA drives.
There once was a time when I'd have been as enthusiastic about a used condom as a used hard drive, but the price of new HDDs seems to have remained obstinately high for several years now and, warranty considerations aside, new drives can and do still fail so I thought what the heck. A "new" drive becomes "used" the minute you power it on in any case