How to calculate/decide on Server infra?

Hey Hashnodes! Hashnoders? Hashnodians? lol

I've always struggled with setting up server infra. I would really appreciate if you could help me around this with your experience and insights.

For example: I need to set up a server for Audiobook app;

Assumptions:

1. 5k users.
2. 500 active users at any given moment. Might peak up to 1k users during the day for a couple of hours.
3. They stream/download a 15MB file every 10min => 1.5MB/min; I don't know whether this is import data to consider or not.
4. The database has user info, audiobook file info.
5. In the future, I would like to have a track of who is listening to what to recommend related stuff. This adds more requests to the server.

Stack:

1. Front-end: ReactJS
2. Back-end: Not sure yet - Nodejs, specifically free plan of firebase instance. This is where the real question is. Is a free plan of firebase enough? If I were to set up a custom server(which I believe I should) on Digital Ocean, what should be the minimum infra?

Hope that makes sense. Please let me know if more data is needed.

Though I am giving an example(assuming it is required), I would like to know generally, what would you consider while designing a server.

Cheers!