I'll try to answer your questions to the best of my abilities.
1. What can you buy the legs out of (legs are the new BoA item coming out in MoP)
They're available from the guild vendor as of right now in the beta.
However, they don't require reputation. That may change.
2. Can monks dual wield, or they just have staffs and 1h offhand?
Monks can dual weild as DPS or Tank. They can also use a staff.
Mistweavers can also use staves or mainhand/offhand.
3. Will I have to use the rogue gear for monks or they coming out with a whole new set for monks themselves?
Yes, similarly to how Enhancement Shaman use Hunter heirlooms. :)
For your healing spec, you'll use leather intellect gear.
An easier way to assess this is to think of yourself as a druid when it comes to heirlooms.
Except you can use swords.
Hope I helped. <3 If you've got any questions, let me know.