How many controllers do you have? I'd have one controller for each person, plus one if you have a guest over because some games allow up to 4 players. Have nunchucks for all controllers. The motion plus controllers are nice because so many games need or accept them now. Rechargeable AA batteries is a must! I found that the controller charges don't seem to hold a charge as long as just charging the batteries on their own. I usually keep 2 sets extra and it's served us well. If you have any young players, under the age of 7 or so I'd suggest looking for a controller made for younger kids. This has been a lifesaver for DD5. Her hands just weren't big enough to manage the regular controllers effectively, the smaller made one she loves, it also comes with the nunchuck for around $30 or less.
We never bought the upgraded sensor bar and honestly I haven't seen the need for it, but our living room is on the small side. If you have a bigger area it might be something that's needed?
As far as other accessories it's going to depend on what games you add to the system. There's a classic controller that us old school types seem to like for the now "vintage" games.

If you add Mario Kart to your collection ( I suggest this one) you will want to look for wheels you can use with your controller. Often you can get a 2 pack of these for $12, I prefer the nerf ones because they are softer. There's other things such as skins to protect the controllers that comes in other colors besides the clear ones provided, guns, golf clubs, bats, boxing gloves, etc....it's endless, you can decide where to draw the line. All games do not require these, but sometimes they are nice to have.
Also if it's in your budget now or in the future I'd suggest the Wii fit plus that comes with the balance board. We love this! The fitness games are fun, not a real workout but it gets you off the couch and mine sometimes sighs when I step on it and I've gained a few pounds.

There are a lot of fun games where you can use this and so far we've really enjoyed having it.
Like any system you can add until you're broke, but my main suggestions for immediate add ones is just simply enough controllers for everyone plus one extra and rechargeable batteries. Past that you can pick and choose what is going to work and not work for your family. Enjoy, we have loved our Wii for 3 years now.