Even if you have the 32GB Wii U console, you'll still need an external hard drive to download the digital version of TT Games' Lego City Undercover.
According to Eurogamer, Nintendo has indicated users planning on downloading the upcoming Grand Theft Auto-inspired Lego game should be sure to have an external HD handy. With the game reportedly coming in at a robust 22GB in size, it's way bigger than anything the 8GB base model Wii U can store. Factor in the 5GB of system updates and a user profile, and you're down to just 3GB of space.
The 32GB deluxe Wii U owners wouldn't fare much better, though there is a chance they could get away with downloading the game directly to the console. After all the updates and such, the deluxe Wii U has 25GB of space remaining. Provided you've done almost nothing else with the console, you could presumably download Lego City Undercover.
The game isn't out until March 19th, so you still have time to think about whether or not it's safer to just grab the retail copy, or purchase an external HD for future Wii U use.