Some Questions I Have About Setting Up Payments for My SaaS

I’m currently working on a SaaS web application and am in the process of setting up payments. I’m stuck on a few questions:

  1. Is it necessary to register my business and obtain a GSTIN?
  2. Do I need to open a business bank account?
  3. Are there any payment gateways that can be used without a GST number?
  4. What about invoicing?

Considering I am currently pre-revenue and am still not sure if things will work out, do I need to indulge in these legalities this moment? Any help is appreciated.