Payment Installments options?

[shakir]

Hi guys. Need some advice/help or if anyone knows of an existing plugin:   

We are in the process of setting up an online christmas gift laybuying store.  To give you a better idea of what we're after please have a look at the chrisco christmas hamper site etc - we want customers to be able to click on what they want to purchase and have the website calculate what their weekly repayments will be based on the 'final payment date' (which will be 4 weeks before xmas).   

ie. if you purchase at the start of the year your payments will be based on say 43 weeks, whereas if you purchase mid-way through the year your payments will be based on say 20 weeks - thus being a higher payment each week.

Payment will be direct debit.

We also need to be able to see on the website how much of each product has been ordered.