Since launch I've been using a Western Digital 2TB Elements External Hard Drive which has its own AC cable (NOT using a Y Cable), and I have had zero reason for complaints.
It's an older model, so you probably won't find the exact same one I'm using readily available, but I chose it because reviews said it was simple, straightforward, no frills, no fancy software or anything, plug and play, etc.
Also, there's WAY more space in that 2TB HDD than the Wii U can even display icons for, so yeah, 2TB is definitely overkill since I'm barely a third into it and already hitting the Wii U Game Icon display limit.
I'd be surprised these days if you had to pay more than $60 to get what you want, and that'd be on the high end. The thing that gives me pause when browsing the newer models though is that many are USB 3.0 and I'd want to read a little more on what exact connection cables they come with and how they'd work with the USB 2.0 ports on the Wii U.