Its hard to tell as there has been a lot of custom work added to this Zaku. But the base kit look like it is the MG MS-06F-2 Zaku II F2 EFSF Version from what i can tell by the chest plate and forearm design and due to the fact that it has movable thumbs. As for the weapons I agree with Ryan that it is a aftermarket/home made gun. However as the 1/100 scale kits are about 6-7in high; 1:12 scale weapons such as the one from the Little Armory line by Tomytec can probably be re-purposed for them.
Hope this helps