New payment gatway app

[3dneonsign]

Hi, 

I am unable to find the required details at a single place.

I wanted to create a payment app for a payment gateway provider. For this what kind of subscription/account type will be required. 

As some are suggested that we need shopify plus account for this which will cost $2500 for a month?

Can someone please help me finding the correct links or documentation for:

1. To create a new custom payment gateway, 
2. The payment gateway will have settings to setup keys and webhooks
3. On checkout page: how to add new option for this new XYZ payment gateway
4. How to redirect user to payment link after the checkout
5. The return url where user will land.

Thanks