Someone take Node.js as the blind way to go, other love Ruby on Rails or the new React, where do you stand?
It seems the path that is being taken is to separate the backend from the frontend completely. Backend as API in the language of your choice (PHP, Java, .Net) and frontend with a React, Node and Redux base.
This helps if one day you want to make an app, you can reuse the API too.
Find a good technical co-founder and go with what he/she is comfortable with.
MVP and 'scale up' are not always to be considered together. In fact, MVP should come way before you even have the final idea for a product, it is about verifying a theory. Then, you might start working on scaling up. Remember you never scale a platform, you scale a website.
With all that said, node.js doesn't fit all types of web development. It will probably be better to look into other dev stacks.
I wouldn't choose Java, because the development is way too expensive. Wouldn't pick PHP, because I don't like the language. Wouldn't pick RoR, because it will kill you when you need to scale up and development is very slow. I personally developed my startup on .NET platform, gets things done fast and relatively inexpensive.
So, not enough information to receive a really meaningful answer. And most of the time it will be a personal preference by your tech co-founder.
depends on technical requirements. My usual fav is Django + React but if streaming is a requirement than Node is a great starting point.