I understand wanting to use your existing USB3 external drives, but as unmesh notes, this will significantly cut performance. The good news, though, is that you can easily pull the drives out of their enclosures and throw them into a NAS, preserving all existing data and eliminating the need to buy new drives.

Personally, I'm a big fan of Synology's products: they are reliable, have lots of features (media servers, backup support, etc.), and affordable.

Take a look at the Synology DiskStation DS411slim—it has slots for four drives, so you'd be able to add more space if you need it later.