For each player, you need to have a controller and a Nunchuk. Each is sold separately (fyi), but you’ll spend around $40 total for each set/player.
The Wii Balance Board is included with Wii Fit (a game title for physical training/exercise). The balance board is used in several other games as well (i.e. snowboarding and skateboarding games).
You may want to consider doing what my wife and I did-subscribe to a monthly GameFly account. It’s free for the first month, and functions like the Netflix subscription system (you order games online through your PC, and they ship the titles for you for rental). You can evaluate the games that way, and if you decide to keep the games, they often charge you a reduced rate for the sale.
We kept the GameFly subscription ($19.99 per month for unlimited rentals), and have used it as a cost-effective alternative to “a la carte” renting ($7.99 a pop) or impulse buying (spending $50 for a game you haven’t previewed is ridiculous!). This has been a good way to find out what the kids like, versus amassing a pile of games that are seldom played.
Be warned-the accessorizing can become addictive! Get one additional controller at a time, and little props like the gel controller covers, the Wii steering wheel or golf club add-ons are a waste of money.