Well, it sounds like you get the gist of what you need already.
WD My Passport 4TB Portable Hard Drive
That was the top recommended for Xbox One for best of space/price ratio(with good performance presumably), you might be able to find it cheaper but that's the best deal I could find at a quick glance £82.99(edit: currently £86.24 on prime, £89.06 without) . Towards the end of last year it was down to as low as £65.99, you have to be patient for deals like that though.
Hope I'm not stating the obvious here too much:
You can of course go cheaper, maybe 2-3TB instead and a cheaper brand, if you're really looking to save. Your search should be "External HDD" and include USB 3.0.