Which modules are best to fulfill the requirements outlined?

I have an idea for a web based service that is more on the e-commerce side of anything really. Certainly not a social networking site or a blog site. My idea is fairly simple and I believe I have all the high-level 'modules' I will require. I am hoping to get input for this forum as to what modules for drupal would meet my requirements.

modules required:
1-account management system - registration, sign-in/out, user profile - this requirement is for users that can subscribe to a service I will provide noted in requirements 2. This requirement seems pretty simple and seems inherent to drupal.

2-subscription mangement system - registered user can sign up for a service, can subscribe to the same service more than once (this would make sense). Subscription service would be paid through pay-pal (silently or otherwise) with auto-renew once per year.

3-email tracking system - for each subscription, an email will be sent out once per x days and must be responded to within y days. If no response after y days, a reminder email is sent out. If no response to either email after z days, a final notice is sent out. If no response after n days, an action is noted internally to be taken. If response has been made, the timer is reset to send a new email 30 days later. - this module should be fully automated and I would guess is one of the more difficult requirements.

4-action tracking system - for all actions that are noted to be taken in 3rd requirement (let's call it a 'to-do', the actions must be tracked; from when the action was first noted to be taken, to being taken. Once action is completed, this 'to-do' can be marked completed. - this module has data generated automatically by requirement 3, a user will mark each action as completed.

sub-requirements:
5-management of all the above system - actions are tied to the subscription, emails are tied to the subscription, subscriptions are tied to account, there can be multiple subscriptions to one account.
6-data management: must be able to store accounts, subscriptions, emails, actions. May also need to store account (credit cards or otherwise) information for renewing subscriptions.

Obviously if it takes 40 modules to do the job then that's what it takes, I'm just looking for some guidance as to which modules would work best. I am new to the drupal framework and I'm hoping to get some help sorting through all the modules available.

Thanks