Some of the parts I used might not be all that easy to come by, but there is a lot of opportunity for part substitution. For the minifig torsos especially, you do not need a full plain torso, but any torso with printing on the front will do, just turn it around so the printing is on the 'back'.
The video explains how it works and might help you see how it goes together if the instructions are unclear in any way.
One note about the assembly process. Depending on the relative orientation of the two crank shafts when you connect the chain, the motion of the legs and the body/boulder may not be in sync. It’s quite easy to adjust this after the fact – just slide the 12 tooth double bevel gear (the black one) to disengage the leg crank shaft, orient the two crank shafts as they are depicted in the instructions, then reengage the gear.
The internal mechanism is the same as for the core instructions, but the central frame is different, to accommodate the surrounding base. For anyone who has already built the core model, unfortunately I don't have a list of additional parts that you need to build this version. But, thanks to user tvroenho, who pointed it out in the comments, rebrickable can tell you which parts are different between the two models: http://rebrickable.com/compare/MOC-3955/MOC-4240