And what should I look for in batteries?

That also kind of depends on the class you intend to run (stock or Mod). But either way I suggest running the b5m with its "shorty" configuration which allows for the use of what is called a "shorty" lipo (much more common and easier to get). And you can only use a 2s lipo in these buggies so that is their cell limit.

As for how much MAH you want out of the battery, I generally go for high capacity shorty packs regardless of which class I'm running. I like anything above 4500mah so this battery below would be a good choice

True Spec Premium 7.4V 4600mAh 85 Amps/90C Shorty 5mm Inboard

I'm pretty sure I'll be running stock. That's what it looks like he's running