I don't know anything about the Wii U, so can't comment on that.
If you get a Wii, you can (as Matt says) buy a bundle which will have, pretty much, everything you need in the box. You may need to get a couple more controllers, though, if you're planning to play with friends.
I don't have any experience of Karaoke or the dancing games, they are my idea of hell, but the Carnival games are good fun, as is Wii Sports (which will probably come with most bundles) and Mario Kart. All good to play as part of a group.
http://www.play.com/Games/Wii/4-/3438343/Carnival-Funfair-Games/Product.html?searchstring=carnival&searchtype=&searchsource=2&searchfilters=s%7bcarnival+%7d%2bc%7b362%7d%2bc%7b10066%7d%2b&urlrefer=search