Enjoy your (?)holiday, but buy your USB external HD when you return.
All external HDs require their own power supply and buying here will avoid a voltage mismatch.
If a warranty repair/replacement is needed and the Atlantic lies between you and the retailer you are in trouble.
As to the best....?
If you buy a ready-to-go, enquire for details first, but why not buy a case with its power supply and put your own choice of HD inside it. That way you know exactly what you have bought.