Ask HN: What tech stack to choose in my first stint as a freelance developer?

I just got my first project as a freelance developer, and I am unable to choose a tech stack. Some background - I've worked with Enterprise Java and C# for a few years now, so I am fairly comfortable with these two languages. But I have never worked with any of the hottest technologies including languages and frameworks like Python/Django, Node, or RoR. I also will need a deployment solution, and I am unable to choose between IaaS like AWS/Azure/GCP or just to go with PaaS like Heroku, since I have never worked with any of them.

Should I explore new frameworks in something I know (Spring/Play/DropWizard with Java) or choose to go with something that I haven't worked with in order to learn new tech as I am developing?