Make sure you include delivery price when doing the numbers as quite often you end up paying more for the postage than the drive itself, particularly for low cost models. • USB 2.0 is sufficient ...
More than three years ago, the Universal Serial Bus Implementers Forum (USB-IF) standardization committee adopted the specification for USB Power Delivery (PD) with Extended Power Range (EPR).
The only case with hard drives where the USB standard matters much is if you connect a drive to an old-style, low-bandwidth USB 2.0 port, which is better reserved for items like keyboards and mice.