Hi! I'm trying to make this recipe, but should I buy pork belly with the rind on it?

I think the choice is up to you. I happen to love the rind any cut of pork. Texturally, it would add yet another dimension to the tacos, in addition to the watermelon and the jicama. I would say "yes" to the rind (it would probably be really awesome in this recipe, especially if you can get it to crisp up), but if you're not a big fan of the rind texture, then just cut if off.