Build a web app to monitor Gsuite Email & send email

I am unsure how much the proper budget for this project would be. Hence putting in a range for the namesake. Please quote as per your fair price.

Looking for a web app developer to build a web application that will monitor gsuite email inbox and send emails to a selected list of clients based on the time that has passed since the last communication. This web app aims to keep in touch with clients and maintain our business relationships.

The web app should have the following features:
- Email monitoring: The web app should be able to monitor our gsuite email inbox and identify clients who haven't been contacted for a certain period.
- Customizable email templates and sequences: The web app should allow us to create custom email templates and sequences that will be used to reconnect with clients.
- Timezone-based delivery: The web app should deliver emails to clients' inbox based on their timezone.
- Automatic inclusion of clients: Each client added to the list will be included in the process automatically.
- Scalable CRM: This web app will be the start of a CRM that will be scaled up to include and automate other tasks in the future. Hence need the web app to be scalable, modular, and built using modern web development technologies.

Looking forward to the best bid.