@Stuart-Martin Maybe an old topic but still of interest to some 
Currently trying to do the same, did all those steps too (came up with some, found others) but struggling when doing pip3 install cryptography which is part of the pip3 install homeassistant process. Did you have any issues with libffi by any chance?
The idea is to have a micro-valid zwave secondary controller. Cheap and very small.
Thanks,
Ka.