OK then, here's a breakdown of patch/update sizes on the games you've listed: Mario Kart 8 ... I have 454MB of update data & 1.6GB of DLC New Super Mario Bros U ... I have no update/patch data Super Mario 3D World ... I have no update/patch data Super Smash Bros ... I have 2.8GB of update data and that's without any DLC Donkey Kong Country Tropical Freeze ... just a small 29MB update LoZ: Breath of the Wild ... 3GB patch/update data without any DLC So that's a quick total of over 6GB in basic updates/patches.
If you're not planning to buy any digital games, I'd say a 32GB model is fine if you're not building a full collection. I have a second 32GB model which has all the updates for my entire 60+ physical game collection (with the exception of LEGO Dimensions which has a basic patch of 6GB, and Super Smash Bros which has a 2.8GB basic patch). If you go the USB harddrive option, the WiiU will re-format the entire drive and then needs to be dedicated for the WiiU (you shouldnt try to swap it around). I had a spare old 500GB PC HDD drive just lying around and put it in a powered case and then attached it to my main WiiU and not had a moment's problem with it in 4 years. If memory serves, the SD Card is not used for WiiU game saves. It is mainly there for the Wii backward compatibility functionality but some WiiU games do have some SD Card features such as Super Smash Bros for WiiU.
If you have any further questions, please feel free to ask :-)