Depends on your child. We have friends who did great with the balance bike. Loved them, learned to ride a regular two-wheeler at three years old because of them. My kids, however, have never liked them. They couldn't figure them out, and they just had more fun on a regular bike with training wheels. My son was a little later than most of his balance bike buddies learning to ride his two-wheeler, but he was still 4, so I think that was pretty good.
As far as helmets, my main suggestion is to get a brightly colored one, because you can see it from anywhere. Also, stay away from licensed characters if you can. I bought my 3-y.o. a helmet with Lightning McQueen on it. Three years later, he still fit the helmet, but it didn't seem right to send my 1st grader to a skateboard camp with a helmet that had Disney characters on it. We ended up getting him a red Razor helmet. It's brightly colored and gender neutral, so it can be handed down to our daughter when she's big enough. :)