I would like to find out if there already exist any APIs etc., to offer such a service. As part of my service: I would like to confirm affordability of users before purchasing a high valued product or service via my platform.
If you're looking to build a service like Xero that imports users bank transactions and balances, you've got a few options.
Xero uses Yodlee on the back end to power their bank feeds. As I understand it, it's rather old school and often errors out, but supports a huge range of banks through scrapers, private APIs, and all kinds of proprietary technology.
You can also check out Plaid - you can think of them as the modern version of Yodlee. They support a more limited number of banks, but have an excellent API and SDK, and I believe are much more affordably priced than Yodlee.