The Wii U Basic console only has an 8GB hard drive, most of which is taken up by the operating system. But as I've recently found out, even the 32GB Premium won't last you very long if you plan on maintaing a decent-sized Wii U library so at some point you'll probably want a HDD anyway, meaning your choice of SKU isn't that important really.
Price is, though, and for under £170 at Base.com you can bag a Wii U, a GamePad, an extra Wiimote (MotionPlus of course), a Sensor Bar, two fine multiplayer party games in Wii Party and Nintendo Land. It's perhaps not the best deal for the core gamer (shopping around for a generous bundle with a headline game might be a better bet there), but if you're transitioning up from the Wii or just new to the party, it's a good place to start.
You'll need a hard drive, of course. I went out like an idiot and assumed a new Western Digital MyPassport would work. It doesn't. But the ten-year-old, Y-cabled HDD from my ancient laptop works just fine. Make sure you get one with either a Y-cable or a dedicated power source just to be sure. The Wii U can be iffy with single cable units. You won't need to break the bank, though, and these can either be salvaged as I did from old machines or you can pick up 500GB units for around £30. Nice find, coconut94!