The last version available on the github repository is a old one, so please wait for the new release before beginning your work.
Yet the cost of the Poppy humanoid with MX motors seems to me, way too expensive to be invested by an individual. I would advice to make this project within a fablab which could handle (at least a part of) the cost.
Also we hope to create a cheaper version with AX motors, whose cost will be maybe more adapted to hobbyist:
The assembly instructions are under progress and will be distribute with the new release:
If you want to start right now, you can use the simulator: