How to create welcome email and post-purchase email with API

Hi,

I'm a developer with short experience of CRM and Marketing.

We want to integrate our application with Zoho email to send:

Confirmation email
Email with a link to trigger account activation
Welcome email
Personalize welcome message
Post-purchase email
Purchase status message (received, in progress, and delivered)

We prefer API because we have application to trigger those emails. But hybrid approach also very much welcome. As far as i know mail service has limited outgoing mail, and i don't really understand how CRM and Campaign can help.

Please help me with answering question below:

What services to use?

A brief process flow

How we can start with free version and the limitation?

Thank you