I would suggest to take up Hard drive space of 150GB minimum for NPM(unlimited)+NTA(unlimited)+IPLSA(50) configuration.
Again I would like to mention that this 150 GB should be minimum for a default historical data retention period according to the Admin guides for NPM and flows per second on NTA.Incase you might want to differ from the default historical settings then you would have to rethink on the hard drive space.
Th 20GB is for the Orion server. The SQL serever will require quite a bit more. See this doc for a better understanding of the requirements and recommendations.