With MoP coming out soon enough, I was wondering if it's too early to prepare and have all of the required heirloom pieces for leveling a monk? I have justice points waiting to be spent and I'm not entirely sure which ones would best fit a monk in the upcoming release. Is this information known yet? Are we dealing with an unknown? If this data is available, what heirlooms should I pick up to prepare for a monk that I'd like to level in MoP?
The data you request are indeed available, since the beta is already alive and kicking.
The monk will be a Leather wearing class, and in terms of Heirlooms it's going to share the gear with Rogues and Druids. Since you can be a DPS, a tank or even a healer, you have the following choiches (I've divided them in "healer/others"):
For the weapons you can choose what you prefer. Monks will be able to use fist weapons, polearms, staves, 1h swords, 1h maces and 1h axes, with the ability to dual wield. As the Druids and Death Knights before them, they're going to be shieldless tanks.