Wii U is fine, but you also want to look into Minute and ISFShax, that adds a layer of brick protection, for SD you want over 32GB if you want to do a full NAND dump of the 32GB model, for other sizes that depends, to run GameCube and Wii games I was using a full sized SandDisk 512GB SD Card, for my HHD it's an older 2TB WD drive I had for years, got it when the Wii U was new, Wii U formats up to 2TB to Wii U format for Wii U apps, I was storing disc rips as save apps on that HHD, the disc dump can be loaded to the HHD as a playable app, so my own rips are slowly being used on that drive, your SD Card needs to be bigger than the NAND to dump all of it, and if you are going to use GameCube and Wii games you need room, the move game types you might run via other apps the more room you need, 512GB is a safe base if you are ripping games between a Wii, as I used it to get disc rips of my games, My GC games alone are around 64GB, it really depends on your intent, but for the HHD I say up to 2TB, but portable dives often need a Y-Cable or a powered USB hub, the SD Card can range based on what you want it for, I wanted my older games on it, and Wii U discs store up to 25GB, so to dump any disc you need over 25GB not that most things are that big, smaller games can be around 5GB, I know I did the math and my discs all end up around 150GB of data on Wii U. Modding is safe, just be smart, and do the Minute and ISFShax.
