Set rules for 'Payment Information' on Onepage Checkout

Question

I'd like to add in some rules for the 'Payment Information' section of the Magento Onepage Checkout.

Currently my site will show the options:

I'd like to only show the 'Invoice' option if they have selected a particular option within the Billing Payment section.

Which file should I edit to access the options within the Payment Section on the onepage checkout? Can anyone point me in the right direction?

No correct solution

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