We are looking to implement a site that offers product subscriptions that are billed annually, but have a 30 day free trial before the first payment, within which time the customer can cancel without being billed.
• The user may be subscribed to multiple products, and may not necessarily start the subscriptions at the same time.
• Billing should be automatic, both after the trial period and on renewal.
• The system should send notice of billing emails 5 working days before the payment is processed.
• Ideally I'd like to use the protex gateway, but can change if this is not possible.
I am new to drupal, but not to PHP, so would be happy to help in development of such a system if it doesn't already exist.
Does anyone have any pointers / experience in this area? There seem to be so many ways of achieving this, I am unsure of which direction to start looking in. I don't want to put lots of time in and then run into problems later on, so I'm asking you lovely helpful people first 
Basically, it provides a framework for enabling people to add recurring payment options to their gateway modules. With it you can add a product feature to the registration product that can specify an initial charge date and recurring billing specs. Then you'll need to write the code in a payment gateway module using a hook in the module that passes the info on to your payment gateway.