I guess the question is 'where to stop' ?
Docker-machine is easy enough as it's just a script, but it has certain requirements as well.. Depending on os etc, will the docker-machine you download match the version of docker installed.. It quickly becomes complex.
I would start by providing detailed 'exception' messages for the cases where a 'auto install' could be an option.
docker-machine not found, but boot2docker version N found. Please download docker-machine version x from url and install it under y...
docker-machine not found, not boot2docker. Please download docker-machine version x from url and install it under y and boot2docker version x for mac from url...
If we could 'log' all those cases to the user and they 'eventually' end up correct, we could consider automating the whole thing in the next step.