You'll be fine with any HDD connection. Most new External HDD's come with 3.0 but depends on your PC's support (look for a blue port).
If you want it as a server might i suggest a NAS drive, this means all your PC's can connect via you home network and you benefit from the faster ethernet transfer rate (if wired connection).
Finally if you do get a HDD, most externals come with a sleep function which powers down the HDD after 5 mins of inactivity, which can be annoying if you want to access info quick, the seek time take a while. An app call NoSleepHD is pretty good.