Checkout customization. Get customer, line item, tax data and redirect to an external url.

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I am building a store for a client and have following requirement. How can I achieve the same?

Add products to cart in shopify, and while checking out, navigate to an external url with customer data, line item data, tax rate data where the user will make payment and then after payment is complete, it'll navigate back to shopify.

I went through various apps available in shopify apps listing but we don't want to use the shopify apps as we have a solution with customer portal and integrated payment solution.

It'd really be helpful to know the options I have, I went through various documention and I couldn't find anything that'd serve client's requirement.