You can put any OS on any hardware, be it consumer or server grade. If you plan to upgrade it you will need to get server grade parts, such as Xeon CPU, ECC Ram (doesn't have to be manufacture branded, just gotta make sure its ECC or Registered RAM), as for HDDs you can use either standard SATA or server grade SAS. Its a pretty good deal for the price.
Check out Dell refurbished computers at Dell Financial Services. Dell gets back leased business computers and refurbishes them and resells them at really good prices. These systems are really well built. Much better than consumer grade computers. I recently bought a Dell Optiplex 790 i5-2400 for $240. It came with Windows 7 Professional. I have upgraded the PC to Windows 10 Pro x64, upgraded the ram memory from 4gb to 16gb and the hard drive from 250gb to 3TB. I have also added a USB 3.0 PCIe card. Eventually I will change the cpu to an I7-2600 when the price drops a little. Also check out FatWallet for coupons. I used one of their 40% off coupons to reduce the price on my refurbished Dell from $399 to $240.