I don’t think Magento bundle will fit your needs then; looks like you will get better results if you develop your own “banking” channel type with appropriate entities. If you want to use Leads, then Sales channel (or B2B as it is called now) will be a better example.
In CRM terms, a banking customer will be represented with Account entity, and their bank accounts/cards will be specific Customer types. I know it is kinda upside-down, but this is how you can represent the 1 Customer – many Bank accounts relation without much hassle.
Then you will need a way to convert Lead into Bank Account (and, if necessary, a new Banking Customer) record when you close a sale. We will implement a very similar Lead → Opportunity+Contact/Account transition in our upcoming release, so if time allows, I would advice you to wait for it and use the latest example.
Other banking products that the customer has can be represented by Opportunities, or you can build different entities of your own for every product—depending on what you want to get in the result.
If you follow this way, your Banking Customer view (a renamed Account view) will display you all their banking accounts and all other banking products grouped by account they “belong” to (or have been sold alongside).
Hope this helps.