member area with subscription

Hi,

I would like to create a protected area accessible only for members who have paid a subscription, and Isotope would be used to provide the payment process.

My idea :
- Add a date field "subscriber until" in the member table
- Create a product in Isotope (6 months subscription - 10 € for example)
- Just after the payment, if the product is the subscription product, fill the date field "subscriber" (for my example : now + 6 months)
a- When the member wants to access to a protected page, check the date

My question :
Could you tell in which isotope file (and which line) the payment is validated and accepted (if possible for any payment type) I searched in "isotopepayment.php" and "moduleisotopecheckout.php" but I'm not sure)

Thanks a lot
Olivier :P