Iirc a modded wii u can run gamecube games so definitely try doing that, some classics on there :)

Get an sd card and a USB stick. SD for homebrew, plugins, and other stuff, and a USB stick if you're getting more digital games. Also if you can, go out and get Splatoon, Mario Maker, Maybe Mario Kart and maybe Smash. Those all seem to be the most active in terms of online play!

search up "wii u modding guide" and click on the first results. here's a tip too. NEVER USE YOUTUBE VIDEOS! USE THE FIRST WEBSITE THAT POPS UP ON GOOGLE. :D