You will need at least 1gb of RAM on the app server (just for Reaction). If you are building your images there, make that 4gb. Yes, that’s a lot more, but that’s because you’re now using the server as a development machine instead of just a production deployment destination. My recommendation is to build a Docker image on your local machine, push it to your Docker registry of choice (Docker Hub, etc.) and then deploy it to your server from there.
As for Mongo server requirements, that entirely depends on your use case. Mongo has some details on that in their FAQ. (but definitely Google it too because there are a lot of useful posts on that topic)https://docs.mongodb.com/manual/faq/diagnostics/#faq-memory