What is the way to identify hardware configurations for small business application?

Right now, I am developing a hybrid application using Java, Mysql, and Angular, it belongs to the public domain (consumers can visit the website and make a subscription type), and anyone can signup/login and view or submit information. (Pages are lightly weighted)

I want to load the page within 3 seconds and launch the small business application over the server (Let's say, AMAZON EC2 INSTANCE or NAME CHEAP REGISTRAR),  What if in the future, at a time 75-100 people are accessing the website resources?

I tried a free EC2 tier instance (Linux) with 1GB RAM, but when I log in with 3-5 users from Laptop (2 different browsers), Desktop and Mobile, etc, the server is responding too late. It takes 5-7 seconds at least to respond to the requests.

so my questions are, 

• How do I identify that I need 2GB/4GB or 8GB Ram for a better user experience? 
• Should I go with a Windows server?
• Is there a way to increase or decrease RAM dynamically on the base of concurrent active users?

Any help to my queries will be appreciated!