If your intentions are to modify your wii, then you would want a external hard drive to store and play the backups off of, an SD card for the actual modification process (as well as running homebrew aps and games from later, although you can also use the hard drive for that, just a lot of times the SD card just works out better) and then everything that comes with the purchase of the wii. I would also consider finding some component cables (you can find pretty cheap online, like 5 dollars at some places) if you are using on a HD TV, which most people are now a days. (Although there is actually an adapter coming out like the end of this month that cost like 50 dollars that will let you use HDMI, but as it's not out yet, haven't really heard any impartial reviews on it, although I personally have high hopes for it). And lastly, if you plan on playing with more than one person, of course buy additional controllers. (The number of games that use more than two nun chucks are very small, and not very good, so I would advise not going any farther than two of those). A classic controller or two is also a great idea, as they are great for many games, including virtual console games and homebrew like emulators.
Hope that helps!