Payment by instalment advice

Hi,

This is my first OpenCart (V1.4.9.3) venture and my client needs a payment system that allows payment by instalments, i.e. pay a deposit then pay the balance on subsequent visits. I need advice as to the best way to approach this in OpenCart. Would it be to modify the Checkout Process or the Payment Module? Are there any modules or extensions that allow this already?

Any advice appreciated.

Regards,

Craig